Adding an adult child to your credit cards or establishing a joint account might seem to be an easy way to repair or boost their credit. But it may not be as good of an idea as it appears, especially if your child has had problems with debt in the past. Studies have shown that three-quarters of co-signers end up paying the loans taken out by their less thrifty credit partners. What's more, if your child misses a payment on a joint card, it will drag down your credit score as well. That may make it tough for you to borrow should you run into an emergency.