Banning curveballs would be ridiculous because then you'll have umpires having to make judgement calls on balls that move. I don't know if you'd necessarily want to put a number on how many curve balls a kid should throw but if they do have a curve in their bag of tricks, they should be able to throw it properly and throw it at appropriate times depending in the batter and the situation as opposed to a steady diet just because 10-12 yr old kids can't hit it. This is where they should be learning to pitch.
However pitchers at 10-12 yrs old who throw mostly curve balls are doing themselves no good down the road if they hope some day to develop into high school pitchers simply because by throwing so many curves, they don't develop their fastball. You can fool 10-12 yr olds all day with the curve ball or something they're throwing at that age resembles a curve ball because they don't know how to hit it. They'd get the same results learning a changeup but all kids like to throw curves.
When they get older, or start playing travel, legion or high school ball, that apprach will fail pitchers because hitters eventually learn to recognize the breaking ball and lay off the junk because often, it isn't a strike and at the same time, their fastball never improved because they never used it and developed it as much as they should so if they havent learned to locate it or failed to add some mph to it because they lived off curveballs all their life. As a result, they never learn how to pitch and they'll get hammered.