When was the last time you looked? I'm not sure it's ever been as high as 20%, it's currently 12% for 1 child, 16% for 2 and 19% if there's 3 or more children.
As BudgetBuster says, that's what he pays, not what the mother receives. If he had 2 children to two mothers, he'd still pay 16% and each child would only get 8%. If he then moves in with a woman with kids, it will be reduced further as he can claim he supporting his "step kids".
So, worked example.
Woman A marries what seems to be a nice man with average wage (say £35k) and has child. She the finds out nice man is having an affair and the OW (woman B) is pregnant. Woman B leaves him when she finds out she was the OW, then 6 months later he moves in with Woman C and her 3 kids.
First the 35k is reduced by 16%, for the step kids, leaving £29400. He pays 16% of that in CMS (£4704), which is split between women A & B, so each get £2352/year or £45/week.
That doesn't even cover half the kids food, let alone be considered payment in lieu of all the additional work, and lost earning potential the mother puts into raising the child.