We did the rafting in Page last year as a family of five. I have three daughters who were ages 13, 11 and 8. I highly recommend it. It was a perfect duration (about 3 hours) and there are no rapids- just calm rafting in a motorized boat - the scenery is beautiful and the kids really enjoyed it. It was August when we went, so we did the early morning one when it wasn't too hot - in fact, down on the water (water is very cold) and in the shadow of the canyon (for at least the first hour) it was actually a little cool and you could have used a sweater, which is ridiculously unbelievable given that the days were about 110 degrees, but it warmed up as the morning progressed.
When I booked on their website, they had the option for a combined trip of river rafting in the morning and the hummer ride to the secret slot canyons in the afternoon so that is what we did. I highly recommend the hummer ride to the slot canyons as well. You get your own private ride in an air conditioned hummer (as opposed to the other company where they ride you out crammed in the back of a pickup truck) and they only take out one hummer at a time and time it so that you have the slot canyon to yourselves for the whole time, which I believe is also different than the other company. The driver of your hummer does a bunch of off-roading tricks and drives up some inclines - and my kids were just thrilled - they loved the ride through the desert and the crazy driving! They provide you a lunch at the rafting place before you head over to the hummer place and it was very good. They did forget to tell us to wear closed shoes for the hummer ride because the sand in the canyon is hot.
One other piece of advice - if you get a chance to before your rafting trip, take the hike out to Horseshoe Bend. It's a small stop just off the highway outside of Page (I got the directions from a thread on TripAdvisor Boards) and it about a 1 mile hike through the desert (we were there in August and all I can say is bring a LOT of water - it doesn't look that far, but in that heat, it was incredibly draining - and the walk back to your car is uphill) but it is SO worth it! You get to view down to the river that you'll be rafting on and it's a beautiful view. Caution though to stay away from the edge! There is no railing and it shocked me how some people went right up to the edge to take photos - the next day when we were passing by it from below in our raft (and in total awe of the ant sized people on the edge where we had just been the day before - it really gives you perspective) our guide was telling us how people have slipped off the edge and fallen (to their deaths obviously) or the fact that the rock can just crumble under your feet.
I highly recommend both the rafting and the Hummer ride - we were on a 3 week road trip through LA, Las Vegas, Grand Canyon, Death Valley, Yosemite, San Fran, Monterrey, etc and this was definitely one of our highlights!