I only have one additional piece of advice to the information from the previous poster. That is, I'd strongly recommend that in the future, if anyone offers to purhase bonds or do any sort of financial dealings on behalf of your children, to be there physically to give the information directly as needed. Not only does this protect your child's private information, it is also prudent to be aware of what others are signing your child's name to.
I don't mean to chastise, nor question the character of your family...but you don't have the advantage of knowing the people with whom they are doing business with.
My family has been impacted by the problem you are fearing most. In an almost identical situation, a relative who had access to my dh's SS# and other financial information did use it to commit credit fraud..and it's effects were disasterous.
Regaining control of your identity and clearing credit is next to impossible, even following the steps highlighted in the previous post. What's worse is if any reported negative action as a result of your identity being stolen you can not just get another SS#. Creditors don't care who is at fault and often will hold you responsible for recovering the money until you have legal documentation (police reports) to prove that a crime had indeed taken place.
Law enforcement and courts see so many of these cases daily, that it's a real all out battle to "get your life back". It takes many long hours and filed police reports before you see progress. It has taken dh almost 5 years to get his credit and good name recovered, and tons of money in legal fees. If the person who has stolen the info is savvy enough, he/she may use your name as an alias if and when they are questioned or charged with fraudulent activity! Getting rid of that is whole new problem in itself.
Don't hesitate to confront your parents. This is really serious! Your child's life could be wrecked before he gets started. And if you think it can't just do a search on the internet. There are many stories of kids finding out their credit was stolen when they were very young when they apply for college loans or try to buy their first car. But because so many years had passed, there is very little they can do to clear their name or credit... and will probably never be able to own a house etc. without a serious fight on their hands.