Move achievements to child account or change adult account into child account

Unfortunately there's no way to do either. Nothing can be transferred between accounts and while a child account can be converted to an adult account, it doesn't work the other way.

You cannot transfer or merge accounts, achievements, etc.

You would just need to replay the games on the new account. You cannot move achievements or game histories from one account to another.

I have my acvount and added on a childs account for my daughter. They would need to recompleye all the achievements but then this new account will do them for thw rest of their lives.

But that would be totally unacceptable
if it can not be done.

It would be small changes to add such a feature and it must be many many situations when this functionality is needed, it
MUST already exist.

I chatted to MS Xbox support.

This is how to do it:

1 create a new adult account and make it administrator of the family

2 create new email address using non-micrsoft (that is not @live, hotmail, outlook.com, instead use yahoo or gmail or similar)

3 add the new email as alternative log in to the adult account you want to make into a child account

4 remove the old email from the adult account you want to make into a child account (only works if this is also a non-micrsoft email, otherwise all microsoft email etc. will be removed from that account as far as I understand)

5 change the age of the new email account (that is your old adult account)

Then I think you can add the old email as family administrator again.