Can't set parental controls on child account

You can't. As for your child you can set up a child account for him and set Parental Controls to disallow the purchase/download of any Marketplace content.

You can't, But I would suggest, setting up a child account, and enable parental controls

support.xbox.com/.../xbox-live-parental-control

Meisun is not correct.

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Child accounts cannot download M or RP content that is free as that is considered advertising. However, child accounts CAN purchase M and RP content if your parental settings allow it. The parental account attached to your child account can manage these
settings.

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support.xbox.com/.../xbox-live-parental-control

It sounds like you just mean the default parent account it sets up. If it has some random name like "PortedOak" or "TrustedOmnivore", that's probably all it is. When setting up a child account (under 18) a parent account is automatically set up to control
what the child is allowed to do. If they can't sign on their own profiles, that is a problem, though.

An account is able to make online purchases only if it is set up as the account of a mature person since the beginning for a minor's account can't purchase online. This is something the service can't control for it's the data that the user has introduced
which make the difference in the way the account is treated. It falls upon you as a parent to set up the minor's account and the parental control over such account.

For more info:

support.xbox.com/.../error-child-trying-to-access-settings