Confusing "Thank You" s

There's a phenomena I've noticed from behind the counter of Joie. When a parent brings in a child, and buys them a little gift, they frequently instruct the child to tell me thank you. This doesn't really make sense to me. If the parent bought the child a little toy, shouldn't the kid say "thank you" to them? I actually did nothing -and half the time a very small child is too shy to even look at me, let alone speak to me. I mean, I did open a store that has fun little things in it that a child might want and a parent might buy for them, so in that vague sense I deserve thanks. But I don't think tiny kids really get that sense of it. In general, I like to, and feel I should say "thank you" to people for buying something and usually do. But, customers clearly also like to say thank you to us. I've definitely found myself in more than one of those "thank you." "No, thank YOU." "No . . thank you" games . . . it's funny.