Which is more kid friendly Yellowstone or Yosemite?

Yellowstone has lots more to interest kids IMO, especially kids who don't hike. The park is very large with a different view around every corner. It has lots of bison, elk, and other animals and birds as well as fantastic and varied geology. Old Faithful isn't the only eruption, there are lots as well as bubbling mud pots, hot springs, etc.

I'm with emalloy. Yellowstone is more interesting, with a wide variety of sights. I think the "naked" earth there is quite unique and fascinating to adults as well as children.

Another vote for Yellowstone. To me, Yosemite is more of awesome beauty and vistas though obviously there's more than that. But Yellowstone has such variety in natural settings, unique environments, and wildlife that I think it can be much more interesting to kids (and us older kids as well).

I saw both as a kid and Yellowstone was more memorable.

Make it another Yellowstone vote. I love Yosemite, but the sheer diversity you get in Yellowstone, from the falls, to the canyon, to the lake, to the gysers, to the wildlife, it's just offers more visual and particapatory kid activities.

And even one more, even though as an adult I can get caught p real quick on the bubbling mud pots, and love the grandeur of Yosemite. But lots more to see for kids at Tellowstone.

I love both but I vote for Yellowstone with kids. Just hanging out in the Old Faithful Lodge is an awesome experience for them. And, if you really have a hankering for a day of beautiful views, you can always take a drive down to Teton NP, which is drop-dead gorgeous. Take them to the Jackson Lodge and go out to the back patio and hear them go "wow!"

It's unanimous so far - Yellowstone. Nothing like seeing big animals rub against your car. No hiking necessary - plenty to see from stops along the road. Also white water rafting of varying skill levels or just a float trip will give them plenty to tell their city friends when they get home.

Yellowstone.

Another thing about Yellowstone is that there are plenty of "walks" - rather than hikes. Places where you park your car and walk about 1/2 mile along nice path to see something interesting. Don't think of this trip as a drive-by Disney-type experience. Park is huge, lots to see and except at a few places like Old Faithful, does not seem crowded (or was not when we went, mid-summer/peak time, a few years ago).

Either place you will want to stay "in the park" at one of the historical lodges, in my opinion. You are already a little late so you need to make reservations ASAP. At Yellowstone, I would stay 1 or 2 night in each of the following- Old Faithful Inn, Roosevelt Lodge, and Lake Yellowstone. We liked the cabins @ Colter Village in the Grand Tetons, but there are certainly other options at each park. You can stay outside the park, but I think you are missing out by not staying in park.

I don't really see how you can go wrong with either place. There are several great national parks that would be good experiences with plenty to do. You have picked two of the best.

If the kids are in decent shape, look out, you will not be able to keep up with them. If they are in shape, take them on several hikes, rafting, kayaking, etc. I would be a little hesitant to let them hike on there on at this age and with no experience. Maybe a short mile or two hike with people on the trail.

We did the typical theme park type trips until our daughter was around 11. We do 3 or 4 national parks every year now and they are just fantastic.

We did the alpine slide in Jackson.

My daughters all time favorite places- Zipline in Durango
Canyoneering in Arches, Canyoning in Oregon, Yellowstone. She doesn't even list rafting the Grand Canyon.

Here is a link to our our Yellowstone/Grand Teton Photos.
http://www.photoworks.com/slideshow/album/A00E72318DFD?source=pw980

Wow - I'll be a Yosemite dissenter here in the event you choose that option. If you do, take the hiker's bus from the Yosemite Valley floor to Glacier Point for the incredible view of the valley, and then hike DOWNHILL back to the valley floor on the Panorama Trail to the John Muir and Mist Trails with spectacular views of Half Dome, and Illilouette, Nevada, and Vernal Falls. We did that hike when our kids were several years younger than your niece and nephew. They still talk about it and the giant Sequoias at Wawona Grove elswhere in the park.

Yellowstone. There are the geysers, the beautiful Morning Glory pool, wildlife. It is an exciting place to be. If you like hiking and beautiful scenery, than Yosemite. If I had to choose, it would be Yellowstone.