The Savings Bond Resource Guide is the one stop resource for all things savings bonds.
Appendix D of the Guide includes the Guide to Cashing Savings Bonds.
It states, "On the back of the bond, have the parent enter a request modeled on the following:
“I certify that I am the parent of Jane Doe with whom Jane Doe resides (or to whom legal custody has been granted). She is 4 years old and is not of sufficient understanding to make this request. John Doe on behalf of Jane Doe.”