I would give the hosting stepbrother more of a pass if the other two kids weren't his. As a childless person, if I had a counter-height or bar-height table hosting elementary school kids or younger, I would definitely put them all at the lower table just for safety purposes. I would have also made the same assumption mentioned by PPs, that of course a toddler would require close supervision by a parent.
The thing is, though, that the owner of the table has the two older kids, and presumably they are permitted to sit at it. In his place, I would have put the two older kids at the high table, and then moved two adults to the low one - I might even have moved two couples to the low table, and had the toddler sit on an extra chair at the corner between her parents, if that would work.
I love the look of a counter-height table, and have been looking to replace my farmhouse table with one, but this thread is making me rethink that idea, since there are issues that hadn't even occurred to me that might come up!