I wouldn’t have considered it a big deal at all. If he held up the line and everyone was waiting that would have been a different story. If the same scenario happened and he got in line behind me, I’d tell him to go first.
He’s a kid remember and if he did something blatantly rude his parent should have stepped in and said something. I wouldn’t necessarily expect a kid to pick up on this type of etiquette. In his mind, he’s probably not thinking any further than staying with his dad.
