Hmmm, so a working professional will be taxed at 40% of his earnings with 4 kids to support, and a single person supporting himself will be taxed at the same rate? But the 4 kids that father raises will go on to work and pay taxes to give the government more money.
Well, it depends on the salary you earn - you are only taxed 40% of anything you earn above £34,370 per year... and considering that 90% of the UK population earns less than 40,000 per year, not that many people pay more than 20% income tax.
Also, if you have kids you can claim child benefit and child tax credits (if eligible) and a single person cannot.
For income tax, if you earn less than £8,105 a year, you get taxed nothing. Then for everything that you earn between that £8,105 and £34,370 you get taxed 20%, and then anything above £34,370 you get taxed 40%.
So, if someone earns £40,000 per year, they are taxed 0% of £8,105 (£0), then 20% of £26,265 (£5,253) and then 40% of £5,630 (£2,252)... so they would pay just over £7,500 in income tax per year,
Then, presumably, the child tax credits (if they are eligible for them) and child benefit will account for the fact that they don't get 'tax breaks' for dependents in the UK (although from what I can tell, the IRS calls the tax break for dependents 'child tax credit' anyway - it's just that in the UK you claim the 'break' in advance and in the US, you get a refund on your taxes when you file them).
In the UK, say you have 4 kids, you would get £3,416 per year in child benefits and then, if you qualify for them, the child tax credit amount based on an income of £40,000 per year would be about £600 (I just did an example calculation for myself, with a partner who wasn't working, and 4 kids) - so that's £4,016 back of the £7,500 paid in tax.
Or, if you earned only £30,000 (one partner working, 4 kids), you would pay £4,379 in income tax, and you would get back £3,416 in child benefit, plus £1,711 in child tax credits - so you would actually get back more for your kids (£5,127) than you pay in income tax each year (£4,379).
Or, if you earned only £20,000, you would pay £2,379 in income tax, and you would get back £3,416 in child benefit, plus £2,823 in child tax credits (£6,239 in total).