Thanks to the wonderful advice I received on TA forums, my family had a great trip to GC, Lake Powell, Bryce and Zion. Here is child-oriented trip report from our family with kids ranging from 5 - 10 years.
Grand Canyon South Rim: Lived up to all our expectations. The adults were moved by the vistas and the geologic history. The kids were thrilled to be in this place they'd long heard about. 3 days/3 nights. Perfect length. Allowed us to do 1 or 2 short hikes/walks plus a ranger talk each day. The Ranger talks are fabulous--make a point of seeing at least one. We were wowed by Ranger Laurie, who spoke about Mountain Lions. Ranger Ron taught us about Condors, another fabulous presentation. My active kids were riveted by these hour-long talks. All of them loved becoming junior rangers.
The kids also loved the Navajo and Hopi dancing that is performed during school vacations and summers.
Their favorite hikes were heading down the steep switchbacks into the canyon. We'd head down for for a mile or two, picnic, then head back up.
The adults enjoyed the Tusayan pueblo ruins more than the kids, who had fun climbing the Desert View watchtower.
The most family friendly meals were in the restaurant in the Bright Angel Lodge.
We loved staying at the El Tovar with its central location and helpful staff, but any location in the park would be ideal. If you're going with kids, it's far more special to stay in the park than in Tusayan. We appreciated having our own car within the park.
Bryce: We only spent one night and a large part of a day, but it was fabulous and, actually, the perfect length of time. TA experts convinced me not to skip Bryce--and I am so glad we didn't. We spent a few hours hiking through the hoo doos, which my kids adored. It's imaginative, interesting and accessible scenery and unlike anything else. I wouldn't choose it over GC or Zion, but our kids seemed to appreciate that it was not overwhelming the way GC and Zion can be. We stayed at the historic Bryce Lodge, which seemed preferable to the hotel options outside of the park.
Zion: What a magnificent place on earth! We all had a wonderful, active time. There are many short, easy hikes accessible by the fabulous shuttle system. This is one well run park, with great explanations of each hike, knowledgeable shuttle drivers, etc! Kids loved hiking along the Virgin River to get to the beginning of the Narrows, and hiking up to Lower and Middle Emerald Pools. They logged hours just exploring the woods around Emerald Pools.
The adults were mesmerized by the Zion-Mt. Carmel approach to the park from the East. It is a must-do drive!
Though the in-park lodge has a lovely setting, we chose to stay in the cute town of Springdale right outside the park so that we had a hotel with a swimming pool and hot tub. Even with chilly weather, this was a saving grace at the end of our trip filled with long drives. There are plenty of places to stay, quite a few with pools, and all with the lovely red cliff vistas in sight.
We all enjoyed seeing evening movies at the giant screen theatre in Springdale. As all the guidebooks say, Zion Pizza and Noodle is fabulous--though can get very crowded. We had a good lunch at the restaurant in the park lodge.
Lake Powell/Page, Az: This was the only disappointing leg of our trip. High winds canceled our Colorado River raft trip as well as boating on the lake.
The slot canyon tour was terrific, and not quite as touristy as I'd expected. It's pricey, but is a unique experience we all enjoyed. We went with Chief Tsotsie; my impression was that all the tour operators offered largely the same services. Be aware that the drive is bumpy with wind blown sand and younger children may find it uncomfortable to sit in the tour companies' open-air vehicles en route to the canyon. Photographers will have fun!
My children enjoyed visiting Glen Dam and learning about its construction.
Lake Powell Resort offered an extremely disappointing experience. Rooms were unclean, fixtures broken, the restaurant's food was subpar, and the front desk sported some very surly service. The clientele was heavily European; my guess is the resort is now focusing its marketing overseas. Plenty of smokers. At times it felt like we were staying in a third world country, without the interesting cultural experiences! The view is gorgeous, but Aramark, which manages this property, could learn a lot from Xanterra, which manages many national park lodges.
General Advice: The national park system is fabulous. We were very impressed by how well it operates, by all the rangers we met, by the lodges, etc. Xanterra too, seems to do a solid job managing park properties. If you'll be heading to several national parks in a single vacation, it makes sense to get the annual national parks interagency pass for $80. Otherwise, you'll pay a fee each day you enter a park. If you don't have them already, equip your kids with cameras for this trip--even if it's only disposable ones. Looking at spectacular scenary becomes far more engaging for them if they are playing photographer like mom and/or dad.