Would you have prompted this kid, who isn't your own, to thank you for a gift, even in front of his parents?

The grandson of a friend turned 9 today. We were invited to his party.
He was opening his presents. The first two were from Rick and me. He opened them, showed interest, then moved on to the next one.
I said, “You’re welcome.”
He stopped and said, “Thank you.”

Well, part of it, I suppose, is that he didn’t know it was from us. Isn’t it standard for the parents to find out who gave what gifts, to make the kid take note of who gave them the present they’re opening? That didn’t happen either, so I started asking, “Who is that one from?”

He still never said thank you to anyone else either, though. No one but Rick and me seemed to think it was strange.

Would you have done what I did, or stayed silent?