The answer is four, BTW. Two adults and two lap children. I've seen this -- a couple with a pair of what I estimate were ~18 month old twins on a flight from either BRU or GVA to IAD. I know it was a pmUA 76-3 which is why the route sticks out. It was a daytime flight, and fortunately a smooth one at that, as they basically turned the aisle seat into a flat fence and let the kids stand up, sit down, play around in the area between that and the window. The kids were reasonably well behaved so I found it amusing more than anything else. But that's very different from your scenario.