I laugh, because I literally JUST talked to all my parents about this. The main idea was "It's SUMMER. We will be outside. We will probably get dirty, sometimes even wet. It means we had fun!"
They may have dirty clothes, despite my best intentions with bibs, etc. They will probably have dirty socks, because I do not allow shoes on the trampoline. But they will (almost always) have a clean/fresh diaper, they will not have food ALL OVER their face, and I bring a pack of Costco wipes outside to handle any serious ickies.