Assuming infertility does not strike, we'll be at our "bare minimum" FI amount by the time we have a kid (we are currently 33/32 and just dipping our toes into the TTC waters, so basically....within 9 months we'll be at that minimum number). Our minimum number assumes some willingness to relocate to a cheaper part of the country and no child-related expenses, so we obviously won't be retiring once our hypothetical baby shows up :) We are hoping to have 2, and in the if-everything-works-perfectly version of things, by the time #2 arrives, we'll be very comfortable dropping down to 1 full-time or 2 part-time incomes. Basically, we want to be done saving for our own traditional retirement and only worry about bringing in enough income to cover our family expenses and maybe save a bit against possible future child-related expenses (e.g., university, if that is still as $$$ and "necessary" as it is now). If that doesn't work out for whatever reason, we'll deal with it somehow, but that's what we're hoping.
We didn't really plan a networth threshold to having kids, but it's just kind of happened that this is (we think), the best time--we've both now been out of grad school for a few years and I've gotten established enough at my job to feel comfortable with the idea of taking a "longer" maternity leave (4-6 months). I do worry about infertility though...it does seem like I run into a surprising number of people who have had trouble conceiving :(