I think MaryL is being sarcastic, with, in my view good reason.
No matter how many children the Royals want/have they will always be financially supported by the state. However, if an ordinary family is now, or may at sometime in the future find themselves on benefits, then the state will offer no financial support whatsoever for 3rd or subsequent children.
How many families are financially secure for the foreseeable future in these days of closures redundancies, even Bank Branches and Post Offices closing and Councils axing 1000s of jobs due to cut backs? These families i.e. most of us, cannot risk having any more than 2 children, no matter how much they want them and would love them. Failure of contraception is not accepted as a reason for having an 'accidental' 3rd child. Is the state suggesting that abortion is the sensible way to deal with a third pregnancy?
If the 2nd and third children are twins then an exception is made but if the 3rd and 4th children are twins then no benefit is paid for either of them, leaving parents to raise/feed/clothe 4 children for the price of two.