In terms of the children, keeping them at arms' length before you shoot with The Force Gun (alt-fire). they love you corner you and pummel. Stasis (AIM+C) is also good to stop them from beating on you for a time.

Your overall mission is to stop the Necromorphs. The idea that have is that Turning off the Marker Signal will be enough. The machine needs a special key that's locked in Rosetta's brain, which in pieces you collect

Fun Fact: the reason Carver (P2) seems irrelevant to the plot is in original version of the CoOp, P2 is meant to be Isaac's imaginary friend, to show Clarke's rapidly deteriorating mental state, but EA scrapped it.

Unitology believes that Necromorphs are the next step in evolution, and that stopping it is against the better of the species as a whole. Propaganda brought about by Marker-touched minds spread throug Humans for centuries.