For taxes, you can opt-in to the IP program. Just remember that the child will need to use the IP pin for life, and the IP pin changes every year (the IRS will mail a new one in December before the start of a new tax season.)
If this is about more than just a relative trying to improperly claim your child, and you are having other problems you can contact the 3 main credit bureaus to put a lock or freeze on your child's credit report, making it more difficult for people to open accounts in their name. Freezing a child's credit report is a little more difficult than freezing your own, you probably have to call instead doing it online. But it can be done.
