D.T.
Stop giving her the option? Don't suggest, tell her to go and that she can't come back to play until she goes. The little body should have something every 30 mins to an hour or two (at the most) if it's being properly hydrated. Make her go and try every 30.
It worked for my daughter and still does and she's 7. We'll be getting ready to go out and I'll ask her if she needs to go and she'll say "Nah, I don't have to.", then I tell her "Go just in case." and wouldn't you know, she'll go. Little goober.
They don't like going because it goes in the way of the fun. Sometimes you just have to make them.