YSK: you may be able to add your children as authorized users on your credit cards.

Why YSK: Some banks have no minimum age for authorized users, and report credit usage for them. If you have a credit card you don't use much, you can add your kids as authorized users, set up a reoccurring payment from the card and an automatic payment to the card, and then forget about it. Your kid could become an adult with an already-maxed credit score.