Well you can look through the VnVs to find out what you want to know. A few things though:

A general can only have four kids. Thus if all the slots are full you won't get any kids until the next gen grows up. Dense trees are worse then sparse ones then since rate is based on individuals.

Looking at the VnVs and the desc_strat:

Brutii don't have a fertility temple which adds to your characters fertility. So they will be fertile or infertile based on chance. They have three familys one of which is full and the other two have only one slot each. Until their are some marrages your chance of kids is relativly small.
Also your oldest unmarried daughter is only 12, and you have only one elegible male. The next oldest is 14.

Compare this to the Julii:

8 available slots in all four families. So double the chance right there. Same number of children but the Jullii have a huge edge because they have more families. They also have an Elegible male.


Hope that helps explain it some. Of course chance plays a big part as well.