You might consider the kid friendly (and 1/4 the price) Los Laureles Hot Springs just west of La Fortuna: http://www.termalesloslaureles.com/

You are perhaps optimistic about the clouds over Poas: an earlier arrival time would be better (also to avoid crowds; when you do, if it IS clouded over, give it 15 minutes). The road (from the park parking lot) down to La Paz will take a bit longer than 20 minutes, IMO (if road repair is still happening; here's a waterfall from the road pic: http://www.govisitcostarica.com/travelInfo/photoLg.asp?pg=1&cid=388&phID=185).

p.s. We like the hanging bridges hike (near Lake Arenal, just past Lost Iguana resort); if your kids have energy to burn after all that.
Itinerary sounds terrific!

We did the ziplining at Arenal, so I'm not sure about the one in Monteverde - make sure there is no age limit (for your 5 and 6 year old), and watch some youtube videos to make sure your kids will be ok with it.

Your itinerary sounds great, wish we were going again!

It all sounds fun to me. I think some of your drive times are a little long. We just went from the Arenal area to the Waterfall Gardens, and it took our driver about 2 hours and 15 minutes or so. A new road has gone in where the earthquake had damaged the old one. I think Tabacon is worth seeing; your kids would love it - very elaborate, and it has a water slide. Reaching Poas by 10:00 is no guarantee that there won't be clouds. We arrived much earlier and it was still so clouded over that we couldn't even see the crater. We lingered around like Sylvia mentioned and eventually they parted, but it took a while. If you're staying at the Peace Lodge, they have a night time frog tour that would be a fun evening activity. Other than that, your kids would probably love just relaxing at the pool. Have you thought about visiting the Proyecto Asis animal rescue center? Kids love that. We were able to feed a sloth, monkeys, and other animals when we went. I agree with x about the ziplines - are your little kids doing that? The one we did didn't allow anyone under the age of 8.

Some of the restaurants we've liked are Don Rufino, La Choza de Laurel, Soda la Parada, Pizzeria de Johnny, Ronny's Place/Mi Lugar, Barba Roja, Salsipuedes, Cafe Milagro, and Cafe Agua Azul. I don't know where you're staying, but both Nayara and the Springs Resort and Spa have good restaurants.

Thanks everyone.

Volcanogirl: I wanted to visit Proyecto Asis but they only have guided tours or volunteering at specific times. I couldn't find anything that said I could visit them anytime I want in the afternoon without making any reservation. Would you know?

Also, we booked Original Canopy Tours for zip lining in Monteverde. Their minimum age is 5 yrs. See http://canopytour.com/monteverde.html.

So it appears everyone is saying that I shouldn't try to do anything extra on top of what I have outlined in my itinerary. I just thought most of the activities such as guided treks, zip lining, etc would end by lunch time so I would have time to kill in the afternoon.