I agree, I think you were right. Either the Mum would have done it herself, in which case she would be grateful. Or in your case, the Mum wouldn't do it herself, in which case (IMO) the child needed to be told that this was unacceptable behaviour! (that's the teacher in me)
I had something similar this morning - the only difference was that I knew that the boy's Mum would have said the same if she was there, cos I know her (but I've never really spoken to the boy). I assumed that because his Mum would also have asked him not to do what he was doing, that he would also respond to my gentle reminder. Not a bit of it - he just carried on, and when I reminded him again, gave me that look you describe. So I said, "you know that's silly, don't do it." (by the way, he was showing off sticking his head through the altar rail, instead of kneeling at it) And he gave me that look again, but did stop.
So even if the Mum is on your side, it's a minefield. But I would do it again.