When you’re TTC a first, you can save up to cover the shortfall in pay throughout maternity leave.
Even if you wait until first goes to school, if you both work full time presumably you still have to pay for wraparound before and after school care for some or all of the days - which isn’t cheap. How do you then save for a second maternity leave with that extra cost? Plus the cost of birthdays, parties, Christmas, days out, holidays, new clothes and uniform, new shoes every few months for #1 which all obviously costs a lot. Before you have one you can choose to save all your disposable income in theory. You don’t have that same flexibility of choice for the second time round - not to mention there are plenty of people I know who reduce their working hours a lot after having just one. This would mean you’d get less maternity pay as well?