Legal Question in Family Law in Illinois

Guardianship is a court proceeding, not a certified letter. You are the parent and have parental responsibilities. You can call the police, or go to court. But beware what you ask for: if it becomes an issue of whether the child is better off in your parents' custody, you could be opening up a can of worms that could backfire. See an attorney and bring a copy of the letter, and explain all of the circumstances and get solid legal help before you do something that could make things worse.