Also its not the specifically 'asking' for a particular thing that makes it rude or whatever. Its the implication that clearly you and your child expect to get something. As London said, no ones going to turn up without a gift unless you tell them to. You should just wait and see if people ask for suggestions. If so then feel free to suggest money (although I just could not do that).
Alternatively if your child doesn't need more toys then do as another post suggests and include info about donating to a charity.
Telling your guests what to bring gives the wrong message to your child.
Sorry, its just how I feel.