Hi there! Have you tried using a different browser or device when adding a child account to your Microsoft family? Using a different internet connection sometimes help as well.
But I would avoid repeatedly trying to add the account for now, as continued attempts may extend the temporary restriction. Give it some time (ideally 24 hours based on my experience), then try again.
When you try again, make sure you're signed in with the correct Microsoft family organizer account, and that the child's Microsoft account is using the correct date of birth. You can also try adding the child from the Family Safety website rather than through the app.