What is the ruling on sprinkling the grave with water every time one visits, and on rebuilding the grave and placing soil upon it after the soil level therein has lowered and it has become exposed? And does the deceased infant see and feel comforted by his parents?

It is permissible to sprinkle water on a grave to firm the soil and prevent the deceased from becoming exposed. However, doing so with the intention of worship or believing it benefits the deceased is an innovation (bid'ah).

Soil may be added back to the grave if its level drops or if rains wash it away, in order to protect the deceased from exposure.

As for the deceased hearing, it is not permissible to delve into matters of the unseen except with clear evidence. Authentic hadiths have been reported indicating that the deceased hear the speech of the living. These texts include all the deceased, without differentiation between young and old.