I lived in Southern Utah for two years, visited Zion and Bryce several times. Both are amazing and if your children enjoy parks and hiking they will love them, but if not they may be bored as there is not much more there. If you go to Zion be sure to visit Springdale, the city in Zion, as it has a lot of cool shops and amazing food. I would imagine you probably did check it out, but just in case, those parks are about a 4-5 hour drive south of Salt Lake City, so plan accordingly. If you guys like Horseback riding both parks have a company (the same one services both, as well as the Grand Canyon) that does horseback tours of the parks. I've done both Zion and Bryce and really enjoyed them both. The company that I used was Canyon Trail Rides.
Only went as far north as Salt Lake City once, and we visited the Hogle Zoo, which is definitely worth checking out. What time of year are you going? If it's during the summer a lot of the ski resorts have cool summer activities during the warmer months and you may want to check them out. We never went to them, never got the chance, but we did a lot of research as we planned to one year but illness forced us to cancel. Seemed really cool and plenty for adults and children alike.