I'd plan on crampons for the top section of both trails. Hermit is steeper so if it's icy you'd really want them there.

There's about 12 hours of daylight at the end of March, plus twilight and a full moon. So if you start before sunrise you should have enough time, assuming your time estimates are correct.

Generally one gallon per person is the usual minimum recommendation for desert hiking. That's for an 8 hour day. Indian Gardens has water year-round so you could refill there before going back up.

I would look at this website to help prepare.
http://www.hitthetrail.com/south-rim-day-hikes

Look at the Hike Arizona website for a lot of trip reports - spend some time looking at the trail descriptions and reading trip reports to see what other people think and how long it took them to do the hikes you're talking about. It will take a bit of time to get through them but I guarantee it will help you figure out if your plans are reasonable or too ambitious.

http://hikearizona.com/traildex.php?SHOW=YES&ID=34&SUB_ID=24&NID=7&STx=3

That links should take you straight to the Grand Canyon trails page. From there you can check out the Hermit, South Kaibab, Tonto and Bright Angel trails. The website is a bit confusing at first - trail descriptions are on the main page for each trail, then if you want to read more reports from various people that have done part of all of those trails, click on the Triplogs tab near the top of the page.

Personally, I would do what you can to mimic the actual conditons you'll encounter when hiking. So put on a weighted pack, hike up and down those 100 foot hills instead of running them, and go on long walks with the weighted pack. A gallon of water is 8 pounds so start with maybe 5 pounds in your pack, then after a month add another 5 pounds to it when you're out walking around.

I can't stress that enough, go for long walks with a weighted pack. Running a 5K is nothing like hiking down and then up those trails, the kids will need a lot of endurance and leg strength to deal with the constant down and then up. The quads/knees/hips take a pounding on the way down. Get some hiking poles to help take some of the stress off. And get real hiking boots with good ankle support.

I think I would just call and talk to a ranger. Sometimes you get one that is super over cautious though.

Different kids are different. It usually depends on if they are having a good time while doing it. My daughter was a bit unpredictable. If there was enough wildlife or enough sights along the way she was fine. But when the scene is basically the same for many miles(like the GC), she would get bored. We have done a lot of hiking and to be honest never really hiked the GC much at all. We have been there 3 times and even rafted a portion of it once(4 days worth). I hope to hike rim to rim at some point.

As for the snow/ice, you may have to wait until a few days until you get there to know for sure what you are facing.

I would do whichever hike you want to do most first. This way if your daughter or you don't want to do more, then you won't feel as if you missed out on the main hike.

20 miles in 2 days with that kind of elevation gain might or might not be that bad. I have twin neighbor boys who are athletic as can be, age 9. They don't mind playing baseball the entire day at tournaments in 105 degrees. Yet, if they go bicycle riding they start to whine after 30 minutes. I have never figured that out.

My daughter wasn't that impressed with the GC at age 7. When we rafted it when she was 13, she enjoyed it much better.

You shouldn't need that much water during that time of year. Certainly have some, but I wouldn't think it would be that big of an issue. I know there is water available in a few places, but not sure during spring/winter if it is on isn't. I would think 1-2 liters per person. Certainly have more for your longer hike.

Personally, I would be suprised if you can make it back up in 5 hours if you suggest-perhaps you can. I'm just basing this on my on experiences on big elevation gains. Stuff like Angels Landing in Zion, Lassen Peak, White Oak Canyon in Shenandoah, Guadalupe Peak, Mist Trail, etc. We usually average about 2=3 mph, but with big elevation gain we do good to average 1 mph. So, I don't know that I would expect an 8 year old to do so. Then again, sometimes we seem to cruise right along. At least it is going to be hot when you are there, so that plays hugely in your favor.

I think you would all fall in the well above average hiker for the GC standards. I see idiots in flip flops and zero to drink trying things like this all the time. It always amazes me.

isn't going to be hot*

Have you ever done any kind of big elevation gains or hiking? It is much different than walking/running,at least in my opinion.

I don't know where you live, but I think I would find 2 or 3 hikes that have a lot of elevation gain and are 5-10 miles and see how you do on that.

I am in Columbia, MO, a relatively flat part of the country. Hiking up and down is a good hint. I have my knees somewhat trained by biking, but again that's not the same as gaining altitude with a pack.

Some people are obviously superhuman: http://hikearizona.com/triplog=12792 -- 13 miles in 5 hours, that's better than my speed on flats... Some people were as late as 10:30 on the trailhead, and still made it within a day: http://hikearizona.com/triplog=71332. Great resource to get an idea of what to expect, thanks @WhereAreWe!

I am in Oklahoma. I hike about once a month in the Upper Buffalo River area several times a year. Hemmed in Hollow is a great hike that is a bit tough-good for some conditioning(I think about 1500 ft gain/loss). You might make a trip there sometime and check it out. While there also see Glory Hole Falls and Lost Valley Hike. Pedestal Rock is also another good hike, but it doesn't have that much elevation gain/loss.

We need a lot more water to get these waterfalls going. If you have never been to the region it would be worth your drive and and day or two stay.

A few more things I thought of:

Are there 2 sets of parents going or just one? You mentioned the other family but only talked about the fitness level of a boy scout and a female that does gymnastics, so I'm assuming those 2 are under 18 yet? I can't tell if the parents are going as well....

Which brings me to another things - be prepared to split up for day 2 depending on how day 1 goes. Some might be sore or just not feel up to a long day. Spiro makes a good point about boredom - some of the kids might get really really bored on a 10 hour+ hike with the same scenery. And the more bored they get, the worse it will be. Have a backup plan for the 2nd day, there's nothing wrong with some of you doing the long hike while the rest of them hike along the rim or just go partway down S. Kaibab with you and then go back up. Obviously much easier with 2 sets of parents than one.

Endurance (physically and mentally) is going to be the key. Your wife understands monotony from training and running a marathon, just not sure about the kids. It's one thing to be in good shape, but another thing to be able to hike for 10 hours without getting so bored that they want to quit. So day 1 will be a really good way to judge whether or not they can handle day 2. I would be really leery of taking first-timers on the S. Kaibab/Tonto/Bright Angel loop without that day 1 warm-up hike to gauge their abilities. If anyone is tired/whiny/bored on day 1, day 2 would be twice as bad.

Just saw your other response: I can't stress this enough, the downhill will be tough on your legs, and that is the hardest thing to train for. You will hammer your quads and joints on the way down, then have to climb out when you're already tired. Completely different than a normal hike up and then down. Imagine climbing a mountain after you just descended it, and that is about what it's like.

I backpacked so had a lot more weight than you will have, but I thought the uphill was a bit easier than downhill. The ascent obviously take much longer but I'd rather hike up and be able to lean into it and use the muscles the way I'm used to, rather than go downhill and keep slamming my joints and putting the brakes on (so to speak).

This was mentioned in another thread not long ago, but in mentioning crampons I think we are really talking about Yaktrax, not real mountaineering crampons:
https://www.yaktrax.com/product/walk

Hopefully by late March you won't need them.

You have received some good advice here. I agree 100% with WhereWeAre, climbing out of the Canyon is actually easier (and safer) than going down, especially with a pack. But the problem as mentioned is you are doing it backwards from most hikes: climbing at the end of the day when you are already tired.

Be mentally prepared to turn around before you've reached your goal, if that's how things are working out.

Good luck and have a great trip!

Thanks for the pointer, @Nelson. I was thinking of renting/borrowing real crampons from more seriously equipped friends, but these chains will probably do just fine.

@WhereWeAre, assessing the mood is why I want to take the first day at an easier pace with a trip doable by everybody. There will be two sets of parents, so splitting up near say Cedar Ridge should not be an issue.

I've had some hikes where I had to summit after some good 10-12 traverse miles. I agree that it was not easy.