“In for a long night” means that time will pass slowly either because of stress or annoyance. In this case, the narrator is saying he’s “in for a long night” because he is surrounded by young children who will need a lot of attention and care.

= going to have a difficult/ tiring/ (and probably) irritating night... We are in for ... = This is what it's going to be like. - We're in for some difficult economic times over the next couple of years.