The seven demigods were on Argo II, along with Nico and... Leo's little sibling. Since the sevens were just a little occupied with killing monsters and trying to destroy Gaea, they left Leo's little sibling with Nico most of the time. Everyone thought Nico would be a terrible babysitter... but the child absolutely loved him. and deep down, also Nico loved them a bit.

Right now, all the demigods were in the common room, when Nico and the kid get in.