If you are wanting to get a savings bond (?) you could use your name and social security number and have the you and the child listed as co-owners, if you use the word (or) not (and) he will still be able to cash it later on down the road without your signature. Or would your sister giver you her SS and have her as the co-owner. Has to be someone you trust though as they would have the right to cash it at anytime.