Once upon a time, in the dim and distant past, a large proportion of income taxpayers were married men with SAHM wives and children. In recognition of the fact that it is harder to maintain four people on one income than one, that man in that situation could claim a personal allowance plus a married man's allowance plus a child's allowance for each dependent child.
That was criticised because it meant that families where men who were paid too little to pay income tax had no support with the costs of raising children. First, then, they introduced Family Allowance, which was paid for second and subsequent children, but child allowance was still claimable against income tax. Further criticism lead to the abolition of Family Allowance and child allowance, and to the modern Child Benefit. It was paid universally partly for administrative reasons but mainly the political one that otherwise middle earners would have less money, plus Family Allowance and Child Benefit were normally paid to the mother so feckless fathers couldn't drink and gamble it away.
(Later on they decided that SAHMs were to be discouraged and knocked married man's allowance on the head too since working wives would have their own personal allowances.)
Then Osborne decided that Child Benefit should be effectively abolished for higher earners, but as others have pointed out messed it up hugely. He and his successors then allowed fiscal drag to remove it from more and more familes, and they stopped increasing it for inflation, transferring the support for poor children into Child Tax Credit and Universal Credit.
Ultimately, though, the question is whether there is a level of income at which income tax should stop recognising that to keep children fed, clothed and heated etc costs money that childless/post-child adults do not need to pay.