I've made the same snappy comments. I was waiting for the parents & kids toilet once when both my girls needed to go and there were three women in the queue in front of me, and their friend using the toilet. Not a child, trolley, pram, walking cane in sight. Poor DD2 was nearly turning purple with the effort of holding the wee in, and in the end I pushed the girls into the toilet in front of two of the women. They made a comment about how rude, and I made a very snarly comment about can't they read the toilet is for parents and kids, so they're the ones who are rude (after making comments to the girls while standing in the line). The worst part was that the general toilets were just next door.

I never have a pram these days and rarely a trolley when I'm shopping with the girls so we try and use the general toilets. If they're desperate then I will take them into either the disabled or parents' toilets if there is no queue. If DH is out with the girls by himself, then he uses either disabled or parents toilets.