"Wouldn't it be best to give this money to mothers who choose to stay at home and bring up the children they chose to have, themselves?"
No I'm afraid I disagree, AOG. It would be best for the Excequer to keep the money and, if there is so much excess available for such largesse, return it to taxpayers by some method. If, as I strongly suspect, there is no such excess then it could be better spent on, say, the NHS. I don't know when or why it became fashionable for parents to expect their childcare costs to be met by the taxpayer (whether they earn £15k or £150k). A simple way to avoid incurring such costs would be to have fewer or no children. This would also help reduce the population and encourage the government to move away from their economic model which requires an ever growing population to fund vital services.