IRS FORM 1310.  If there is no 'will' and you are not going to get a 'court appointed or certified letter of representative', then your only options are the following; to be filled out on IRS form 1310.

1. Fill out Part 1; check box C.

2. Fill out Part II; 1 = NO, 2a = NO, 2b = NO, 3 = YES.

Fill the 1310 (signed and dated by the parent) with his last 1040 to claim the refund. I have filled it this way on one time before and the only time the IRS asked was for a death certificate about 3 months later. The refund was finally released to the parents of my clients.  

NOTE: This worked for my clients and when NO will exist, NO court appointed letter and it was a child of the parents. I can't speak to whether or not this would work in another situation.