The worst possible situation would require that you bring the Mac to them along with its original, dated sales receipt. This assumes your child did the unthinkable which would have been to invoke Lost Mode in addition to changing all your credentials (login name, password, Apple ID and / or its password).
You may be able to recover use of that Mac on your own, without having to ask Apple to help. Start with If you forgot your Mac login password - Apple Support. Methodically examine all those possibilities, all the way through and including If you can't reset your Mac login password - Apple Support. They eventually conclude with contacting Apple Support which this site is not.
Should it be necessary, depending on just what your child did, read If your Mac is lost or stolen - Apple Support. The very last suggestion is to
... my first comment. Just do everything else you can first.