In the opinion of some, a properly thrown curve won't hurt a young kid. BUT, BUT, BUT unless you can guarantee me a kid will throw the pitch perfectly each and every time they take mound, don't ever have a kid throw a curve, dead wrist, live wrist or somewhere in between. NO kid that age can repeat their movements each and every time properly and perfectly.
The ONLY reason coaches have kids throw breaking balls at young ages is because they want to win (duh) without regard to player development.
If, however, your goal is more noble and appropriate and you want to develop pitchers, have them throw fastballs, change-ups and show them how to vary speeds and get a little movement with finger pressure. You may not win every game now but that kid will be a better pitcher because of it.