Okay. So. I see y'all are taking things a bit too literal.

My thoughts on the whole "Why did Jack think Felix did something to the kids" take I'm seeing a lot now:

First off, Jack was concerned about Felix's safety as well. You could tell from the tone of the voicemails. His first priority was his kids, though. He's not in the wrong for his focus being on his children instead of the adult who's supposed to be taking care of them.

Jack doesn't automatically assume Felix has "done something" to his kids. I don't think for one second that Jack actually thinks Felix did anything intentionally. His voicemails gave no indication of that. By the time he arrives at the house, he's scared and most likely has a gut feeling that something has happened. He knows his best friend. He knows if something has gone wrong, Felix will probably be too scared to confront him. That's why he's begging him to tell him.

Plus, he didn't jump to any conclusions, his kids have been gone for three hours well into the night since Jack said they were meant to be home. Any parent who actually gave a shit about their kids would begin to panic by then. No calls are being answered, and now he goes to the house seeing no one's there. He's desperate. He's rambling by that point because he's terrified. He even starts jumping to denial, trying to convince himself it must be some bad prank.

Just because he says "Felix if you did anything to those kids, I'll fucking kill you." Doesn't mean that he thinks Felix did something intentionally, its just how a scared parent would speak to the adult he trusted his children's safety with. "if you did anything" means if the kids got hurt under Felix's supervision, it's just Jack placing the blame on him, not him thinking Felix actually did something to them.