See the additional cost of adding another child.
A second child does not cost what the first one costs. Housing, heating, a car, subscriptions and much of the food shop are already being paid for and absorb another occupant at little extra cost — that is the shared share. What does not share is childcare, which scales almost linearly per child and is the reason the marginal figure stays substantial despite the economies. Hand-me-downs shave a further 8% or so off clothing and equipment. The share-of-first-child figure is the honest headline: typically 60% to 75%, not 100%.
Marginal cost of an additional child
Marginal = (current cost per child x (1 - shared share) + extra childcare x 12) x hand-me-down factor
Commonly 30% to 45%, concentrated in housing, utilities and transport. It falls once a larger home or car becomes necessary, which is the step change that makes a third child costlier than a second.
Because it is priced per child by regulated staff ratios. Sibling discounts of 5% to 10% exist but do not change the structure — two children in nursery cost close to twice one.
Across the whole budget, yes. They can cover most of the clothing and equipment line, but that line is a small fraction of the total once childcare and food are counted.