You are at the local playground with your toddler for the 1st time. She goes to the sandbox and processes this new area. Another toddler comes around. He's been to the park a few more times and is possibly a year older. The boy takes sand and throws it at your child. He comes to your child and wants to wrestle. His parents are cuddling together on the parkbench watching their son's actions and not doing a thing about it. What would you do?

Personally, I would sit down with both children and I would say (very nicely to the little boy) "we don't throw sand at our friends. We can all play nicely, though". I would then stay near the sandbox to intervene if it happens again.
In my thought process this avoids confrontation with the other parents or the impression that you are judging them for their parenting (or lack of).
It also provides a gentle lesson to the little boy about appropriate behavior, and to your daughter about how to handle someone who's misbehaving.

I would tell that kid that that wasn't being nice. If he would like to play with my child there will not be any throwing of sand, rough play. If that boy doesn't seem to understand I would tell my son that the boy doesn't seem nice and doesn't want to play nicely and we won't play with him. Following that I will take my child away if that child doesn't leave.

Yes, if the little boy doesn't stop I would definitely take my child to a different area.
Of course, inside I'd be steaming that my child had to move because the other parents weren't dealing with their child.
Is there anyone who would say something to the other parents or do you think that would just invite conflict?