By the time Donna Peterson's three children are grown, her estate will be worth $5 million. The kids -- now 5, 8 and 16 -- can cash in on the family assets when they turn 30. But there is a catch -- each must sign a prenuptial agreement before saying "I do." No prenup means no money until their 55th birthdays, says the Chelmsford resident. "I'm not saying they're going to make a bad choice, but it happens," explains the 45-year-old, who has been happily married for 17 years without a prenup. "At 21, with puppy love, who knows how...