How to deal with an orphan?

I'm not going to pretend I know exactly how this adventure is planned out but I like to think there is a possibility for at least 3 branching paths. I'm also assuming the parents aren't perfect and may in fact illicit some inadvertent reason for the players to murderhobo them.

The players resolve the conflict non-violently, somehow reuniting the orphan with her parents one way or another.

The players could put the parents behind bars or just straight up convince the parents to take back their kid.

The parents are murdered out of cold blood or out of principle.

Like if the players bust in determined to kill the parents because they left their child an orphan, then of course the kid will cry if there was no due course for justice to be discussed between the two parties.

The parents are murdered after convincing evidence has proven them to not worthy of the world.

The use of evidence doesn't have to be objectively correct either. If both the players and the orphan are convinced then the orphan won't be as sad, or may be happy depending on what happened.

There's endless possibilities and how the orphan could respond, but that's the beauty of play: we don't know until the moment occurs. Do what you think makes sense for the entire table with the information you know. Failure is okay so long as everyone understands and accepts how it got to that point.

EDIT: for other more foreseeable happy endings the players could find some infertile couple who would love to take in the orphan girl. This would probably alter/add a challenge in the second half of the investigation to help the couple construct a convincing lie for the orphan.

The point is it's not your job as the GM to know the beginning, middle, and end of an adventure. Rather it is to present the situation, help and challenge the PCs to move towards their intended goal, and hope for the best.

Not really helpful side comment: I can't see the word orphan without thinking of The Pirates of Penzance. A whole pirate crew made of nothing but orphans, who won't attack ships that have orphans on them. Could be some inspiration, at least!

Play to see what happens. Figure out who the parents are, what their motivations are, and why the girl is orphaned. Then just let the players decide if they take the girl up on her offer, and if so, what they do when they meet the parents and talk to them.

Maybe the parents got addicted to drugs and sold the girl off to get a fix and she doesn't know it. Now they're clean but have no idea where she is. Do the PCs get angry and kill them or tell them they're going to make sure their daughter never sees them again for what they did?

Who knows. That's why you play to see what happens.

If you decide that the parents were secretly demons disguised as humans who were going to eat the daughter if they find her again, your players will almost assuredly kill them.

If you decide that the girl was separated from the parents during an orc raid and the parents were driven off somewhere and never found their way back to her, it's going to be an easy feel good reuniting story.

So you have some control over what feelings may be evoked when the PCs find out what the parents' deal was. Make it as complex as you want by deciding what caused the parents and girl to separate, and what motivations the parents have, and then play to find out what happens after that.

[removed]

What would Conan do?

If the child is without parents you find someone to care for the child.

Your parents hurt people and they died because of it. I'm sorry.

When the players find her parents there is the possibility for them to be killed (maybe even by the players themself).

Make sure it won't happen.

I don't want to end the adventure with 'The girl cried when you told her that you killed her parents!'

Well, she may curse them and swear revenge.

...only to return a few adventures/years later as a highly skilled assassin. :]

Can someone give me advice?

Sure! More details would be welcome though.