This is a long (10 miles!) but technically easy canyon. I think an active 10 or 12 year old could do this, but it does take ALL day and you need to get an early start. The closest airport with easy access to rental cars is St. George, Utah, about an hour’s drive away. There are numerous shuttles in Springdale that can take you up to the drop off.
There are two or three small drops (10 to 15 feet) that require a rope. We brought harnesses, gear and a full rope which would be best for kids, but I’ve heard of plenty of people that bring a 50 foot rope and hand-over-hand (not for kids) or just tie into the rope and get lowered. It really isn’t very hard technically, but PLEASE do your own research and find out more on Orderville canyon and whether you feel comfortable doing it with your kids.
It starts out hot and dry, and ends up wet and fun. The bottom part is called “the waterpark” because of the small slides and pools that must be negotiated (very easily). It also ends up joining with the Virgin River canyon so you get to hike part of that famous canyon.