How to add Apple Wallet to Child under 13 yo?

I don't think anyone here will be able to answer that for you.. you could contact Apple with the link in the upper right of this page if you wanted to.

I didn't really find much when poking around.. but the phrasing used in several places leads me to believe it's not necessarily Apple just deciding for you.. the comment 'or equivalent minimum age depending on jurisdiction' is used in several places.

Gives me the impression that there might be something legal about it?

Legal - Apple ID and Family Sharing Disclosure - Apple

or just leave them feedback

Feedback - iPhone - Apple

I can't answer that question. This is a user to user only forum. You need to speak with Apple for clarification. I also suspect Chris0107 is likely right and it has to do with legal age, which I'm sure Apple MUST follow.

A child under 13 can have their own Apple ID, but they cannot have Apple Wallet.