I didn’t know this: the number of children under 5 in Washington State has remained virtually flat for the last 12 years. Just take a look at the graph to the right (drawn from official estimates by the state Office of Financial Management, here).
That’s surprising, given that the total population of the state has grown by more than a million over the same period—roughly a 20% increase.
Part of the explanantion is a slight decline in fertility rates (the number of children per woman) over the last decade, led by a steep drop in teen pregnancy. (More on that here.) But it seems that the bigger reason has been demographics: the baby boom generation is aging past its peak childbearing years while the much smaller "baby bust" generation born in the 1970s has moved into them.
I’d expect to see a rise in the number of kids—with an attendant increase in the burden on schools and the like—over the next decade or so, as the children of the baby boomers (the echo boom) begin to enter their most fertile years.