The only way to avoid the snake den of coordinating efforts of competing agencies (an effort that is doomed to fail) is to have them independently access the same signal. There is no need for competing agencies to coordinate. There is no opportunity for gamemanship.
Some might argue that certain necessities cannot be purchased in fractional quantities. Housing for example. One might go into a community and find no units available for less than $400, therefore, housing assistance in any amount of less than $400 is meaningless.
This argument ignores the effect that government transfers has on pricing. If the maximum allowable government rent assistance in a given community is $400/month, you can damn-well-betchya that there will be no units available at a lower cost. The cost of housing will find its fair price if, and only if, the government gets out of the voucher/directed payments business. Give the recipient their monthly stipend as cash and let them decide how much should be allocated to housing and groceries and other things.
The prime decision point on the part of the mothers is when they opt to become pregnant. This supposes, of course, that everybody knows what kinds of activities lead to pregnancy.
Since this is an entirely new way of doing business, it should be widely publicized at least 12 months prior to activation. As a matter of fairness, it is my personal opinion that we owe the mothers that amount of lead-time. They should know what they are getting into.
Granted, this would slow down the full implementation by 18 years. It would also create the confusion of households where older children are "paid for" at a higher rate than younger children.