Adding a child or spouse as an authorized user to one of your credit cards, also known as piggybacking, can help them build credit. Of course, that's as long as you continue to maintain a low credit utilization while making all your credit card payments on time. Miss a few payments or drive up your credit utilization ratio by carrying large balances, and you can do some major damage to your authorized user's credit scores too.
That is, unless the credit score is based on the information contained in an Experian credit report. Why? Experian, Equifax, and TransUnion all include authorized user accounts on their credit reports. However, unlike the other 2 credit bureaus, Experian doesn't include any "negative" accounts. It only includes authorized user accounts if they are positive, which means you shouldn't ever have to worry about an authorized user account potentially hurting your Experian-based credit scores.