The actual mechanics of affecting the change you desire is not clear to me. That's more of a legal question, not an income tax question.
I'd expect that you would have to take your name off the accounts as custodian or transfer the assets out of the account to new accounts in the children's names. (That's the "mechanics.") The children would then gift the assets back to you; generally that would entail a closing out of their accounts and transfers in kind into your account. There could be gift tax issues, depending on the amounts involved and whether you're married or not. I guess as long as they understand their rights there's no problem.
