For me, I think there is a difference between rubbing a child's butt and feeling a child's butt. I know that when my children were babies and toddlers, I used to do the same thing. I don't remember doing it when they were past the toddler stage though. What I do remember is gently patting their butts as they passed toddler stage. Even to this day, I will pat my 9 yr old grandson on the butt (gently) now and again when he comes to me for a hug.
If you mom rubbed your butt along with your back to calm you and there was no other "touchy feely" thing going on with that, then I would say if it doesn't bother you, then it was ok.

sabby