Is it possible to switch the gamer tag associated with the parent account to a child account?

Thanks for this suggestion. I have gone to cast my vote on this. It is absolutely something that needs to be looked at, because I think a lot of parents were unaware of the multi-account capabilities when they got their first console...

[...]So I won't be able to make the parent account a child account, for the family safety features?

There is
an idea on the "suggestion box" website Xbox Ideas that asks for exactly that. The more people vote for that idea, the more likely it is to get implemented.

While this won't be of any help to you right now, it might be possible in the future to move your "adult" account over to your child's kid account.

But as the others already said: Currently your kid either has the option of starting all over again with their own child account, or you'll have to live with a kid that is regarded as being a fully grown adult with no child restriction settings.

As for the purchased content: As long as you set the console your kid is playing on as your account's "Home Console" (you can check that by signing into your adult account on the console, pressing the Guide button and in the settings under "Personalization"
check that this is your "Home Console"), your kid can play all the games that were purchased with your adult account, as the "Home Console" will share all your account's games and Gold memberships with anyone playing on that console.

Only the game progress on that account can't be shared.

Also beware of the "Home Console" setting having a limit of 5 Home Console changes on your account every 365 days, which means you can only change the Home Console for your account 5 times during a year.

Thanks. So I won't be able to make the parent account a child account, for the family safety features?

Okay then. Thanks for clarifying.

Hey there Tamiya, you cannot switch gamertags from one account to another.