YABU, you should have planned better.
My Mum has a samll shop, and as it sells childrens things, she is sometimes asked if children can wee. Health and Safety is a big issue. If children have to go into her stockroom and through to the toilet, they risk tripping over things, or things falling on them. There is a LOT of stuff piled up in this tiny stock room, and if anything happened, she would lose her livelyhood because she's not insured for the public to go back there. The risk is small, but would you think the same if she didn't bother to have public liability insurace in the shop at all.
Also, on the odd occasion that she has allowed it with customers she knows well, the parents have allowed their children to wee on the seat and haven't cleared up, so she has ended up having to doing it.
Then there is the thing about it being a small shop, where she is often the only staff person there. Wherever she puts herself she could be burgled from, if she stays out the front people could steal from the stockroom, if she goes back with them, for all she knows, someone could be waiting for her to go out the back so they can steal from the front.
Even in bigger buisinesses people have jobs to do, I don't see why parents think they have a right to take people away from the actual jobs they should be doing to acommodate their bad planning. I can't remember ever having to do this with either of my children.