Some parents make handsome money offers payable upon the grandchild's birth.

Planned Grandparenthood's carefully written literature would detail all the joys of life grand-child-free a calm living room, extra money for luxuries during the golden years, etc.

If your timeframe is long - if you're thinking in terms of benefits for your children or grandchildren - then this effect quickly makes it unreasonable to spend any money for their benefit.