Pay it to the family. They have their parents' rights yet, don't they.
Children have vastly lower living expenses than adults (or more correctly - the basic marginal cost of adding a child to a household is negligible).
There is no reliable way to ensure that income is actually spent on that child.
It is apparent that some parents (for want of a better word) already see children as a benefits-magnet.
This gives a viable alternative to working a productive job and earning a good wage: pump out 9 kids by the age of 21 and enjoy a sweet $100k salary until they come of age themselves. You may safely neglect them in the meantime and pass any costs thereof, such as incarceration or hospitalisation, onto the state.
I think a much better approach is to limit a basic income to adults (or those financially independent of guardians) and give every child a Finnish-style baby box.
http://www.bbc.co.uk/news/magazine-22751415
Limits the financial impact of a child, and has very minimal opportunity for abuse.
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.
"or more correctly - the basic marginal cost of adding a child to a household is negligible"
Seriously?? Are we talking about agrarian society or about modern countries?
You won't have your child in a studio, so naturally you'll need a very different home that costs (or rents for) a very different amount of money. And it all goes downhill from here. Mom or dad need extensive period of staying with their child off work, then find a babysitter and pay her, I'd say a first child accounts for more than +50% of a family spendings.
"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.