Beware the kiddie tax that REWahoo's source referred to. Any investment income by a minor above a certain level is taxed at the parent's rate. So the tax return is filed on behalf of the child, but it may be at your rate.
I'd also be careful about trying to transfer that money back. It may be possible, but it's meant to be a one-way transfer.
fairmark.com may be a better source than this board. UGMA & UTMA Custodial Accounts for Minors
Tax programs such as TurboTax probably also cover the tax question, and so should a professional preparer.