What else was wrong? Were there tire tracks on the driveway? Some ashes left in the fireplace? Was the trailer on the next site over decorated with swoops in garish non-earth-tone colors?
I see nothing wrong with kids (or adults) using sidewalk chalk on the sidewalk/cement pad of their campsite, assuming the results are not obscene or somehow inappropriate for polite society. There are plenty of things that other campers may do that some would find tacky or worse; I find some of the signs and "ornaments" and hanging fish-shaped awning lights rather distasteful, but apparently the owners enjoy them and it's their vacation just as much as mine. Even if sidewalk chalk drawings are not my own preference, they're harmless and nondestructive and evidence that someone else had a proper fun time.
Camping near curmudgeons, on the other hand--that can be a trying experience...thankfully, I haven't had to deal with that at all frequently.