A curveball can be deadly if the kids can't hit it, but you don't want the kid reliant on a pitch that 12 year olds can't hit well because it curves. I agree working on the fastball changeup combo is the most important thing.
I learned the hard way that if you don't have a good changeup you will be relegated to the bullpen, that's not good unless you are one of the top relievers, i.e, setup or closer. (Luckily the slider FB combo works pretty well out of the pen)
It's a good idea to try and learn the curve just not at the cost of the fastball changeup.