We did Sedona with our 11 year old this past April, Easter week. GO FOR IT! We all had a good time. The trip was part of my mission to show the family there is more to US than Orlando -I'm pretty sure you may know the desire!
Weather: I too was concerned that it would not be as warm as we desired, but it was absolutely fine. I'm not sure if it is the elevation or not, but the sun is HOT and we were in shorts and tshirts by day and just threw on a sweatshirt at night.
Kids Activities: Jeep tour (the rougher probably the better for kids), Grand Canyon day trip (we did a tour from Sedona), Out of Africa Wildlife Park (just a visit, probably ok and it was very hot that day to walk around - but we splurged on a VIP tour that as excellent), Horseback riding at Dead Horse park, renting dune buggy type vehicles to explore off road trails.
My DD gave a thumbs down to trolley tours - they were a bit boring for me too, but gave me a good lay of the land for our own excursions.
Place to stay: We did an exchange via II into Hyatt Pinion Pointe. I would highly recommend for two reasons. The resort itself is impeccable - the location is right in the heart of downtown. All tours left from the area. We didn't even use the car the first few days.
As for comparison to San Antonio, we stayed at the Hyatt there also. DD had a better time in SA, but mainly due to the resort (not the area). Can't beat the slides and lazy river there and we also did SeaWorld, underground caverns and a drive through animal park.