Ok, I'll give you a new straw man here and fess up to not really feeling the pinch at the moment. Together we earn just over 100k gross, live in a two bed ex council terrace, run one car for my commute and DH takes the train. I don't think we are much worse off with one child than we were without.
Yes, we paid nearly £1k a month from 6m to 3y and since then we are paying more like 600/m. I worked out how much tax was saved from this earlier in the thread. Come Sept I am likely to be on ML again as DS starts school, but when I go back to work we will either juggle our working hours (possibly both going 80% of full time) to be there at the start / end of the school day or pay for childminder and breakfast club for him and FT care for DC2. But the expenditure we have so far foregone by being parents (i.e. the drinking money, extravagances, holidays) more than outweighs the nappies.
I doubt DS makes an appreciable increase to our heating, washing, food bills. Breastfed exclusively until I went to work, then weaned on the same food we were eating, plus morning and evening breastmilk. Have never bought a jar of baby food and could probably count on one hand the baby ricecakes I may have grabbed in an emergency. I've been tight enough sometimes to give him the mini jug of milk you get with a pot of tea in a dept store cafe.
We drink less booze (at home and out) go to fewer concerts, restaurants, clubs, etc. We go to restaurants as a family (have done since he could sit up) and he eats tasters from our meals and drinks water. Have only ever used the "children's menu" at pizza express. So rarely have babysitter costs. We think more carefully about the value for money of a holiday and mostly don't bother. We have spent little on activities like softplay because of already having paid (through council tax) for public parks, nature reserves and playgrounds, so we get the use out of those. We buy a lot second hand - toys, clothes, buggies.
We have the same size house as we would without a child. We have sunk 40k into extending it, from 1 to 2 reception, 2 to 3 bedrooms. I've no idea whether this will be reflected when we sell but it will enable us to live here very comfortably with 2 DC as long as we need to. We'll therefore move when job economics says so, not family size.
I think (as my epic post comes shuddering to an end!) that my point is for working parents the only real cost driver in having children is childcare, and it increases more than pro rata for subsequent children. Everything, absolutely everything else that I can think of, has cheaper or free choices available. Take Childcare Costs From Gross Pay, I say, but I can't see this being done other than through linked tax self assessment of both parents (no matter how much under HRT threshold the second earner makes) and therefore lose the principle of independent taxation (which we already have with the CB grab)