WIY, what does the local municipality say about the matter? Do they object? If an employee from the relevant department would see a kid do this, would they care? Do they actually condone this or simply tolerate it? Or do they forbid it?
I think the validity of your assertions really depend on the answers to these points.
If this is an approved activity for kids, I don’t see why your opinion on their art is relevant, and these parents are not “teaching their kids to be mazikim” if it’s an approved activity.
But if kids chalking the sidewalk is not approved by your local municipality (perhaps even if it is tolerated), however, then I agree with your points about defacement and chinuch.
Having said that, even if it were approved, if it is true that those kids would then have less respect for public property, then I would agree that parents should probably just say no for chinuch purposes.