For approximately fifty years after the Civil War, a popular way to memorialize young children who had passed was a figure resting in a half shell. Prior to 1900, twenty-two percent of all children in the United States died before their first birthday.
The shell can represent a pilgrimage, spiritual protection, and innocence. Using those meetings, it makes sense this became a symbol for child graves.
Wealthier families would employ sculptors to make one that represented their child. Poor families, who wanted their children memorialized, adopted the shell as a way to mark graves when Sears Roebuck offered them in their catalog.
Here is a great academic article about these monuments by Annette Stott.