Future children
From Kokorodatabase
Technically, future children are defined as any offspring of a character, in play, deleted or existing in the canon universe, that comes from a time period after the current setting. It is under debate whether the character has to be a child or not in existence in the current setting (see Kiki Reikaira, who was born during the run of Kokoro, which is played in realtime-- although, not being any PC's child, this character does not fit the definition in the first place).
Some people believe that the label "future children" implies going to the past and visiting their parents, and prefer the term "next-generation" for games set entirely in the future. At the moment, this definition of "future children" encompasses both. Please vote on the talk page by clicking the "discussion" tab.
Future children became popular during the first summer of Kokoro's run, in 2004. Shirogane Masume was retconned to become a future child, and was the first of the Shirogane children, which became the most popular class that year- to the point where applications for new ones are barred even today.
Mirror Mew, FTMM and Kingdom Chronicle are games where the cast is almost entirely made up of future children.