A 9 and 12 would average a 10 or 11 child. You may get another 9 or a 12, but it'll most likely be a 10 or 11.

9 + 12 = 21

21 / 2 = 10.5

Therefore, the average is equally split between 10 and 11, making either child equally as possible. But if I were to make a guess on which one you'll get, I'd go for the 11. It seems the higher number is more prevalent than the lower.