I have to admit, and maybe it's because I'm adjusting still, it does seem that cost of living here is a lot harder to deal with. When you start thinking you could retire to the US comfortably on your UK wages...yeah. I think the fact that both my kids started school over here almost immediately and after no formal education in the states helped make it more of a growing pain financially..the school lunches, the uniforms, the activities, etc. etc.
As for starting to have kids, I have to assume that the feeling of not having enough money will not go away any time soon, at least not before menopause, so perhaps it's better to use other barometers: how easily it would be to have mom at home, how busy dad is, if a move is coming up, etc. I think there perhaps should be an established minimal living standard though; i.e. if you just can't squeeze formula and diapers into the budget for the foreseeable future, maybe it's best to wait a bit.
Using hindsight, I would also be sure to have help available should something really bizarre happen during pregnancy/childbirth. I was lucky to have family around when things went awry for me, but as an expat I would have no such comfort (well, at least not until the in-laws warm up to me more *shudder*).