I have found through my research that a remaining parent is granted guardian rights by courts for their children if the other spouse has died. So that would mean that most likely William McMullan would be the parent of William Parker McMullan. Of course if both parents are deceased then other family members usually were granted guardianship. With both names being William I would bet they are father and son. The child's mother was probably deceased at the time of grandfather Gray's will. Check the census records for that area to see if both are listed in same household.