The fence thing was what I was going to suggest. That's got to be pretty awkward sitting in your pool and those 2 kids standing there staring. Obviously you wouldn't want 2 kids in your pool that don't even know how to swim.

You can't put someone's kid in "time out". That could really start a war. But why does it have to be a package deal? If the daughter is being a brat then tell her to go home..tell her why she needs to leave and let the brother can stay. What can the parents say or do? Make you watch both the kids?