Did he steal your key? Or did you give it to him? If he stole your key, he wasn't entitled to your house. But if you gave him the key, he had entitlement to it.
If this were a friend, not a kid, you might be able to sue her for throwing a party in your house without permission. You would not be able to get her charged with breaking and entering because she overstepped the conditions that came with your key.
Presume they had intent to water and steal at a felony level when they entered.
If they had to break open the door, entering would be burglary.
Using a key they were supposed to have, to enter a building they were supposed to be able to enter? I would say it should be treated like the former case, not the latter case.
US law may not always agree with me, and apparently there are states where shoplifting can count as burglary. But I say stretching the definition that far is ridiculous.