I completely agree with this comment. I had a similar (but not identical) circumstance in one of my games.

The party was looking for leads on a dragonborn hellbent on destroying the town because some hero from the town in time past killed his “father”, a powerful red dragon.

The party went to a rather run down tavern and wanted to see if anyone had seen anything odd recently, and the bard (also the son of the tavern owner) was singing a song about great walls of fire appearing in the mountains. They questioned him, rightfully so, and decide they want to bring him with them and ask him more questions. One of the town outcasts, a blind monk, steps in and tries to ward the party away from chasing the dragon.

Things get heated, and they wind up in a fight with the monk. They end up killing him after he gets violent, they then panic and try to run out of the tavern with the bard in tow. The owner comes out from behind, hearing a commotion, but the party sorcerer casts sleep and aids in their escape.

Now, as of this moment the party is guilty as the same crimes as OP’s party: murder, and kidnapping. Granted the party didn’t have a host of 50 burly armed people that were witness to their acts, but there were guards that began investigating. The party, realizing that they dun goofed (partly because I told them that the town would be hostile towards them if anything happened to the barely post-teen), wanted to return him. They decided to use illusion magic to make it seem like they saved the bard from some orc that had kidnapped him, and then returned the bard to a thankful father.

This party was generally good, but in a very morally gray zone. Yes, they then persuaded the father to sell them his tavern for a handsome amount of gold, and yes they were investigated by the guards, but because of out-of-table context I gave them they were able to keep their characters out of the “hey let’s kill a kid” zone, and only in the “hey let’s commit fraud” zone. And yes they had killed the monk that was originally supposed to help them with his warnings, but given how he wasn’t a major npc I didn’t want his death to derail an entire campaign, so I ruled that the guards didn’t entirely care about his death since I had put him in a combat scenario with the party in the first place.