I have let my 5 year old go by herself when absolutely necessary. I stand outside the door and she looks in the bathroom before she goes in. If there's someone else in there, I try to wait until they come out.
I am really glad that a lot of public places now have family restrooms. Makes it a ton easier when I am out without my wife. Plus, some of the guys rooms I go into are soooo nasty, I can't stand the thought of my daughter having to go in there.