According to Jim Recap: Slumber Party
When Cheryl goes down sick just before the big birthday party, Jim thinks that he can step up - how hard can it be? Little does he know what he's getting into. That big black notebook Cheryl carries around is just Cheryl being Cheryl, right? Probably. But he also tends to miss some details along the way, like those nine little girls aren't coming over for a couple of hours and then going home - they are coming over for a slumber party. That's right, all night.
Jim and Andy start off slowly. Really slowly. So slowly that it looks like they would be better off by getting Cheryl out of bed, in fact. Then they seem to go backwards when Andy pulls out his card tricks and Richard Nixon impersonation, and it's not getting any better when he pulls out the blueprints to explain drywall to the girls.
Then Jim comes up with the idea of telling a ghost story to get the girls scared. That works pretty well, they get scared, but someone ends up calling 911 and the police come. When he tries to find out, the girls tell him only that it was "Jo" - last name "Momma". This isn't working, and then he finds out that Ruby is hiding under the sink because things are going so badly.
At that point, Jim realizes that he needs another tactic, and that's when he and Andy succumb to girl power, letting themselves be brutalized as dress-up dolls, horses, and whatever the girls may want, and that's when things start to take off. Before long, it's a party to end all parties, and things turn out for the best.
