Do not assume that because a child has a guardian appointed for him that the parents of that child are dead. A Maryland relative in 1817 gave money to two grandchildren and then appointed a guardian to oversee their inheritance. The parents of the child were still alive and their mother was the daughter of man writing the will and were not named as the guardians of their children's inheritance.
Given the era, the writer of the will was likely trying to avoid the son-in-law having any control over the grandchildren's inheritance. Giving the money directly to the daughter in 1817 would have realistically meant that the son-in-law controlled it.
Giving it to the grandchildren and appointing someone else to oversee it took care of that.