“… Jizo takes care of the souls of unborn children and those who die at a young age. Children ‘in limbo’ in Japan are said to go to a place called Sai no kawara, where they must create piles of stones into small towers. But every night the stone towers are destroyed by demons, so the next day the children must make new piles of stones.
“The making of these towers is to help their parents accrue merit for their own afterlife. This is why you sometimes see stray stones that have been made into little towers alongside Jizo statues. People make them for the souls of these children, to help them achieve their goals. People also leave toys, candy or fruit as offerings at the base of Jizo statues.”
– The Japan Times: A Guide to Jizo, Guardian of Travelers and the Weak, by Amy Chavez, March 31, 2012
From the wiki: “Jizo Bosatsu is the patron kami of children, expectant mothers, travelers, and aborted or miscarried babies. It is one of the most beloved of all Japanese divinities, working to ease the suffering and shorten the sentence of those serving time in hell; to deliver the faithful into Amida’s western paradise (where inhabitants are no longer trapped in the six states of desire and karmic rebirth); and to answer the prayers of the living for health, success, children, and all manner of mundane petitions.
“Statues of Jizo can sometimes be seen wearing tiny children’s clothing or bibs. Grieving parents place toys and other offerings beside the Jizo statue to invoke his protection of their dead child. Offerings are also made by parents to thank Jizo for saving their children from a serious illness. In the Edo period, when famine sometimes led the poverty-stricken to infanticide and abortion, the mizuko kuyō [fetus memorial service] ceremony was adapted to cover these situations as well.”