As much as we'd like the parent to step in and teach/change their child's behavior, I feel it's completely our responsibility to deal with it while in our care. Parents aren't always on the same page as a caregiver is, so many times they leave it to us anyways.
I'd keep the 2 children separated, keep them focused on different activities. Or another thing you could try is teach them the 'kill 'em with kindness' technique. Keep asking each child what they could do for the other one that would be helpful, kind, nice, etc. Like adults, kids need to learn to get along with others and your kiddos are certainly old enough to start grasping that. Tell them, if they truly can not get along, then find someone or something else to do.