I do not know if the plinth shown in the last photo was originally part of this grave. These are at most 4 person plots. The plinth and its celtic cross may have been moved onto the grave at a later date.
I have looked at the Church burial records and the only possible burial would be an infant, Thomas Kingsman.
I have added his name and dates here though he may not be related to the Simmons family. The green button leads to his Familysearch page.