This is a brilliant, but underutilized idea. If your son is under 18 and gets a girl pregnant, you are responsible for child support payments until your son is 18. Currently this is only applicable if the mother of the child is on welfare. This is part of the Welfare Reform Act, specifically known as the Parent Liability Child's Act:
Another significant change implemented by the Welfare Reform Act is that parents of a noncustodial teenage father (the grandparents of the minor-mother's child) are liable to pay child support until their teenage son emancipates, if the minor-mother receives welfare. Prior to enactment of the Welfare Reform Act, grandparents were never liable to pay child support for their grandchildren, and the government could not collect child support from a minor-father until he became employed.
Unfortunately, this is not mandatory, it is up to each individual state to decide to implement it or not, according to this part of the law:
Enforcement of Orders Against Grandparents
Under the federal welfare reform law, states are allowed – but not required – to enforce child support orders against the parents of a minor who is the non-custodial parent of a child if the teenage, custodial parent is receiving TANF benefits.
If the part about the minor mother having to be on welfare were omitted, and this law were made mandatory, it could quite possibly have a positive effect on the currently bleak child support enforcement outlook experienced by too many custodial minor mothers.