When logged in a Kid account, clicking the "Change" button in Settings → Appearance → Custom CSS shows the "General" setting page instead of the expected one.
I haven't seen a way to set it from a Family Admin account either.
There also doesn't seem to be a way to set "Customize widgets visible in your search results" either from the Kid account or a Family Admin account.
(I don't think that should matters, but for information, we have AI Question Answering disabled in the Parental Controls).
There should be a way to set these things, either by the Kid or by a Family Admin (or both).