The customary drink for the Hina Matsuri festival is
Amazake. It is a sweet, non-alcoholic version of ‘sake’ (Japanese
alcoholic beverage made from fermented rice; usually served hot), that is
prepared from fermented rice. A yummy soy sauce-based soup is also served
on the occasion. It contains clams shells. Clam shells in the food are supposed
to be the symbol of a united and peaceful couple, since a pair of clam shells
can closely fit to each other.
here (Hina Matsuri Recipes) for the preparation of Amazake.