Pretty dress & outside play

[gcj]

I see where you're coming from and agree HOWEVER being the mother of a 4 year old girl who since she was about 2 1/2 refused to wear anything but skirts and dresses, I get the mom, too. My daughter wears what looks like nice dresses, but have either been given or bought 2nd hand and I know it only fits for a very limited time, so I don't care one bit what happens to it. If you've warned the parents and they continue to send her in dresses then they've made the decision to live with the dirty consequences and probably don't care either.
That being said, I do tell my daughter that she has to at least put on leggings if not splash pants to play outside if it's mucky or just not that warm. And she's mastered how to place her skirt/dress over top of splash pants.
If they don't provide what she needs to play outside, I would let it go. If they're really annoyed by it, they'll either send her in pants or send pants for outside.

Sincerely, the mother of the girliest girl I've ever known!