This sounds like a perverse incentive [0]. If you reward impoverished people for having children, it is not unlikely that the net result will simply be more children birthed into poverty.
And, this touches on a broader conversation about UBI. IMO, payments for the kid should start in the third trimester. Our society does not value stay-at-home parents nearly enough. It's a job, and a damned hard one at that. Doesn't do much good to give people a guaranteed income that's too small to raise kids on -- you'll end up with parents working multiple jobs and the cycle of poverty continues.
What insane arrogance. Do you have evidence or is this some gut feeling you're having?
Or "common sense"?
Or because you said so?
Edit: accidentally a word.