Shiro Sake is very sweet white sake that is very low in alcohol.
It is made from sweet sake, distilled spirits, rice malt and steamed glutinous rice.
A a milder version can be made from Sakekasu (the solids or “draff” of rice left over after making sake), water and sugar. You boil it, then strain. When serving, add a bit of fresh grated ginger.
Shiro Sake used to be served at the Doll Festival holiday to girls and women. This was traditional since what is called in Japan the Muromachi Period (1336 to 1573 AD.)
It is not served much any more to children, though.