Your ex is just an asshole! It would help if you told your lawyer about the situation. Be sure to explain the situation to the child. And, of course, contact your local locksmith and change all the locks. You can give the spare keys to someone else. Your ex doesn't deserve that kind of trust, especially after trying to manipulate a child. This is a mean and unworthy behavior of an adult man. I hope your children will support you and understand. It seems that their communication with their father should be somewhat limited. Talk about it with a psychologist.

As others have said, change your locks. Give somebody else you can trust a spare key for emergencies. He's free to do the same, but that's not your concern. Tell your kid exactly what you've done and add that's there's no pressure or expectation for the information to be either conveyed or hidden. Just as it's wrong to use a kid as a messenger, it's also important that the kid is explicitly aware they're not being used, saves them going through unnecessary confusion and guilt.

Yeah. That needs knocking on the head. Change your locks

No wonder you divorced him. "Surrender" his key, and change YOUR locks.

He’s an asshole. You never use kids

Two can play that game. Have your attorney call his attorney and have his attorney set him straight. If he wants to use go betweens, might as well do it up the right way.

Sorry to hear that. Hugs