Nothing. I finished the challenge and sometimes I would get like 9-10 boys in a row and then a bunch of girls in a row but in the end after a hundred kids I ended up with 49 boys and 51 girls. It evens up after and sometimes it's just luck of the draw. Sometimes it helps if you delete the same gender townies. Like if you have a ton of men in your game, you might get girl babies as the girl tries to compensate or vice versa. But overall it'll even out and five in a row isn't that many, we had that plenty of times throughout the challenge