B.K.
When kids are playing on playground equipment -- and on a playground -- they need to be in gym shoes. It's just safer. She didn't intend this e-mail to offend you or insult you. She's trying to keep kids safe and that's a good thing. Personally, I think kids should change into gym shoes when they go outside to play if they're not wearing them. Any other shoes aren't really appropriate for running and playing.
And, yes, some people have to be TOLD to do certain things with their kids. You might not be one of those people, but trust me, there are a lot of clueless parents out there.