Everybody have vastly different living expenses. Accounting for that expenses is the thing we're trying to avoid when replacing every kind of pension and benefit with one BIG. Why would we want to revert it 180 degrees when talking about children?

The whole point of what I wrote is avoiding the use of children as benefits-magnet by withhelding part of their BIG into savings. Pumping out 9 kids won't work because you'll get just 3 BIGs for them - the rest will go into savings. Then, having a child or two will actually be very financially rewarding because it's what is good for society.