It seems silly on the surface, but it is also possible that the judge had never heard of Steve Irwin (a minor celebrity in his day) or did not make the connection to Bindi.
Also, it might be routine to require proof that the parent signing either has full custody or is the only surviving parent.
The more troubling part of the story is that apparently just because she is a minor the details of Bindi’s contract became public record. It is nobody’s business what she is paid.