It makes no sense that a human baby has a higher hd than adult commoner (or expert, wizard, sorcerer or rogue). But just going by humanoid from MM is not right. There is a huge difference between humanoids so obviously a child is not going to be some of the strongest humanoid. I checked all my 5 MMs and non of them had human non-classed. I suggest you just make them level 1 commoners, with severely reduces stats.