"If a couple wants kids" - we have a problem right here. How is it suddently "if"? Somebody gave birth to both persons in that couple, and to their parents, and before that - why would they be the ones who break this chain?
They should surely have kids when they want to do so and they should not be put in disadvantaged position by having kids - which, well, does happen in almost every developed country and in many less-developed ones. Therefore, BIG for children.
Surely they should also work, but you know, their employer doesn't care whether they have children or not, so it should be compensated somehow.
Heck, I think women should receive BIG from the month their become pregnant. Delivering the baby is very financially-intensive process, no doubt.
In addition, as several other commenters pointed out, if you increase the BIG per child, you risk enabling people who have large families just to get the money. That much, much worse than asking people who want kids to provide for their own.
Nobody asks whether you want to support the ill, the disabled and children, you just do that. You cough up cash because they are already there and you have no alternative.
You really don't.
I've recited my solution of preventing people who have large families to live off their children, it's kind of sad you don't bother to argue with it.
There's also some choice involved in whether you are making enough money to support your children -- but not that much.