Ditto regarding calling your HR person. Although rates may go up next plan year, they could tell you what you'd be looking at this plan year.
I have an educator's state plan. Our rates are such that I pay a set amount for DH, a set amount for 1 to unlimited children, or a set amount for DH + 1 to unlimited children. So, we'll be taking the big jump in premium with LO#1. After that, insurance cost stays the same no matter how many we have.