I agree a four year old should be allowed to go,and they would be at our school. Older children are not stopped from going, just encouraged to go at breaktime...we can't make them!! I was just trying to show what happens as a result , and didn't make myself very clear. I had a 9 year old have an 'accident' this week, and sorted her out straight away. That was because she left it too late before asking to go to the toilet, then because she couldn't get her tights down quick enough at the toilet, ended up wetting herself. It was no problem,there was no crossness,(is that a word?) it was sorted quickly and discreetly.
Sometimes I just wish parents (and I have 3 children myself) would try to see things from the school's point of view.
I confess,though, I am baffled that the child was left in wet clothes,unless they actually had no changes of clothing left to give out,(some parents never seem to return borrowed items), in which case they should have phoned you to fetch some in . Or maybe it had only just happened? I do think,though, if you know your child has a problem, you should send spare clothing,just in case.
If it really was the staff's fault i.e. they knew and chose to do nothing, then that definitely needs complaining about,preferably in writing, to the head.