someone compliments your childrens skills or accomplishments but the child isn't there?
If the child is there obviously the child thanks them, but when they aren't saying thank you just seems silly.
For example: my son plays baseball. He is an amazing pitcher in his age group. He's 9. Often other parents will come to me and tell me how great he is, which is awkward. I take no credit for this, so a thank you is inappropriate.
It's just weird.