First, you must be very proud of both of your children. They chose to do the right thing, even in these circumstances and that is a testimony to the values you raised them with..
As a parent, I can understand your concerns regarding their safety. I am the wife of a police officer, and we deal with gang violence here where I live too. My husband said, if you contact the dectectives handling the case, they can put you in touch with the prosecutor's office, and they have special handling in cases for witnesses when there is a fear of retaliation. So Monday, call the detectives and also the prosecutor's office and explain your concerns. They can offer some assistance to your family so that there is less likelihood of retaliation.