As the previous post said, the child's soc. sec. number is not required...the purchaser can use their own ss#. I have purchased savings bonds for my niece and nephew using thier ss#'s, but obviously my brother and sister-in-law trust me with that information.
As a little aside, someone once told my BFF that she could not use her ss# to purchase a bond, but that isn't true...you only have to read the back of the form to have that clarified immediately