Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
I think I lucked into a solution. I tried going on to family.microsoft.com.
I didn't have any options the first reply detailed.
Instead I ended up messing about and clicked on my son's 3 dots to change his profile picture. It came up with the password option and forgotten password. I have my email email set up as the recovery option so I received the security code. This allowed me to change my son's password.
I don't know if this works for everyone but it did for me