When will Microsoft remove the digital content I refunded?

Hiya!!

First off, if your 5yo is able to purchase digital content from XBOX, you should be setting up your security procedures to prevent that from happening.
You would go to setting>account>sign-in, security & passkey. Select sign-in & security preferences> select Ask for my passkey> from there you can select a custom passkey.
Next, you can remove any game from your library to prevent further gameplay.

Happy Gaming!!

First I Suggest is that you go into you Microsoft account and check if he has repurchased the digital content again because when you refunded it. since you download the game they will just basically be a waste of space because when you try to start the game
it's should redirect you to the Microsoft it purchase it.

To make Sure that your In control of what is being purchased go to setting>account>sign-in, security & passkey. Press A on Change my sign-in & security preferences> go to Ask for my passkey> put in you custom passkey. by doing this the next time he tries
to purchase anything without your permission will require that passkey.

I downloaded Minecraft for him, he just downloaded (bought) a loooot of worlds/skins/etc., those are what I refunded. He can still access those worlds and skins though!

Are there other account on the console that purchased minecraft? If that the case then someone basically saved there receipt on your console also know as game sharing.

I already double checked - he has not repurchased anything, I had immediately added a passkey for purchases as well.