I guess they're referring to things like kids running away from distant relatives who want to kiss and hug them, which is definitely something that happens.
That's definitely part of it, but I think (and I also remember) that a part of it is that children have a well-developed sense of personal space and who they are close enough to for different types of contact, from a pretty young age, but their sense of closeness is (quite reasonably) derived from actual (remembered) contact rather than from having a genetic relationship or contact before they can remember.
We have to remember that kids don't have any reason to love distant relatives, and that adults don't have the right to demand physical affection from kids.
