Just like people, cats have different tolerance levels. And she obviously recognizes the baby as "the kitten" of the family. Should an older child not from the immediate family try it, she'd most likely bite or scratch. Although I have seen cats who are really craving attention/affection and will be up with too much, just to have it.
Our littlest cat, Andrea, a rescue is very obxnoxious & fierce-playing with Andy & JC and they put up with so much. I try to supervise as much as I can because they will cry with pain, yet not defend themselves. at least Joey gives an excellent hiss & a good boxing about the ears and Andrea gives him so much respect... guess there's a lesson to be learned there