According to Jim Recap: The Nanny-Cam
As Jim and Cheryl decide to go out for the evening, it dawns on Jim that both Andy and Dana have plans. Andy is headed to the science fiction convention and Dana is going on her 17th date with Ryan (the 18th if you stalking is included). This means they don't have a babysitter for the evening, and since their last babysitter isn't avaiable (she died), they don't have anyone to watch the kids!
The surprising thing here isn't that there isn't anyone to watch the kids, but that Jim actually noticed this before it came time to leave for the night. Perhaps even more surprising is that he is concerned about Cheryl's choice of babysitter - a lady she met in the park, who was apparently watching some other kids. So Jim, naturally, does what anyone would do - he buys a Nanny-Cam! Of course, he isn't quite that good, and he somehow cons Andy into putting up half the money for it.
They set it up in the kitchen, and after watching Andy play light saber with the mop (and break a teapot, for which he blamed the girls), they catch the new babysitter doing... absolutely nothing. They do find Cheryl's secret stash, however, and they come up with a plan to take a little money each day and see how long it takes her to notice. Of course she notices right away, and since it happened when they hired a new babysitter, she thinks that the sitter is responsible, so she buys a Nanny-Cam! And she catches her husband and brother stealing money, and comes up with a plan of her own.
Cheryl and Dana act out a small play where they pretend to be burying $15,000 in the back yard, which they know Jim and Andy will see on their Nanny-Cam. When they get home later, the entire back yard is torn up because the guys haven't been able to find the money - because it doesn't exist. Oh well.
