As I illustrated a few posts ago, a single parent on 35k with 2 children in full time nursery (in a fairly cheap area of the country), would have £600 left per month after paying for that childcare. If any of the people saying that those earning 35k should not get tax credits can pay their mortgage, food, petrol, utility bills with £600pm then I would like some tips 
The poster who says she earns 50k, brings home £2,600pm and doesn't get any tax credits - if you also had to pay £1500pm in childcare fees, then you would get some tax credits (I'd estimate about £300pm). If you don't pay for childcare, then you don't need tax credits, just as the woman on 35k would not need, and would not get, tax credits once she is not paying for childcare.
Tax credits ensure that everyone has an acceptable level of income, after the costs of childcare are paid for. The alternative would be that the woman in my example would have to give up work while her children were small, and then likely end up in less well paid employment when she did go back. Meaning that she would probably end up relying on tax credits for much longer than she would if she had continued in her career.