Depends on the circumstances and the age of the children.
I'm a bit worried about the 'allow' part, I wouldn't suggest it, but sometimes you can't stop it. Like today, my 3 year old decided he wanted a wee, pulled down his pants and wee'd against my car tyre, on the school run, in front of everyone including the headteacher. Did I allow it? Well I didn't give permission exactly, but neither did I dive on him and try and stem the flow.