Not a problem for the U-11 playing up because all the Cup that they could be playing in will be over.
BUT your right about a U-12 player playing-up could still be playing in the Championship Cup. Good

but I don't have an answer for that problem.
Maybe If its with the same club just ask the coach. With another club (changing to a new club) go to the try-out and ask before hand. But remember that those setting around doing the registration are not always the ones who know the tough questions.
Tryouts to early create tons of problems :x