John
It was certainly not uncommon to put a child into a grave which was open for another burial at that time, however in the case of Ann Reidy the grave was specially opened for her burial, which would have incurred the cost of the sexton/gravedigger, also I would think that special permission would have had to obtained before opening up a grave. Because of this I can only think that little Ann was the child of a relative or friend of the family buried there.
Regards
Gillg