Q: Child's apple id

Unfortunately no. He may not like it now, but imagine how he will thank you when he is 30....

Here is the part of the Apple Support article that indicates that:

Enter your child's birthday and tap Next. The birthday for a child's account can't be changed after the account is created. Be sure to enter the correct date.

Here is a link to the article FYI:

https://support.apple.com/en-us/HT201084

Sorry

GB