Long time ago when we had a passport for my 2 year old ( who is now 28) I was told to sign his name and sign my name above his with mother in parenthesis.
Either that way or what the previous poster said should be acceptable. Even government officials will understand that a little kid isn't going to sign their own passport.
"My child is too young to sign his/her own passport. How do I sign my child's passport?
In the space provided for the signature, the mother or father must print the child's name and sign their own name. Then, in parenthesis by the parent's name, write the word (mother) or (father) so we know who signed for the child."
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