Isn't that "Kleincousin" in German? "Younger" could certainly match the "Klein" part. That would be the daughter of your cousins, so there's surely a male variant too.
I'm German and I never heard the term "Kleincousin". According to Wikipedia [0] it seems to be used regionally to refer to 2nd degree cousins (which I would call "Großcousin").
Anyway, "Kleincousin" and "Großcousin" don't imply the age of the cousin, but a degree of relationship (with regional differences to what is actually meant).
To refer to a younger cousin, I would just say "jüngerer Cousin". "kleiner Cousin" may be possible too (like "kleiner Bruder" for a younger Brother), but it sounds a bit like childs talk and it may not be immediately clear to everyone what is meant.
