I have costodial accounts for my kids for the same reason you do, to make them save, birthday $ and such goes into that account. When the account gets to $500 I "make them" buy a savings bond. It goes in their name and they can get the money later. The costodian should be the parent that can do the banking with them. Keeping the account below $500 makes me feel better that it isn't really enough money to do anything really stupid.
I also have a brockrage account for the kids that they have nothing to do with. it will be theirs when they reach 21, if I ever tell them that it belongs to them. that is the real money for the future.
-L