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.