There is a practical limit on how many wireless connections you can have into one router, but you typically won't run into that in a home environment. Just make sure, as Terry said, you turn on the encryption. You'll need to do that at the router, then at each wireless connection. You can set up the printer to be shared along with files on each of the computers, but that is a windows thing, and has little to do with the network. Keep in mind that your internet connection is being shared between all of the connected computers, so if each of the kids are trying to download the latest movie or music, it will be slower for all, but they'll still be connected. Moving files from one computer in the house to another doesn't affect the internet connection speed.
On some of the new Directv dvr's, if you have the right win operating system, you can play videos and show pictures or music from attached networked computers. Neat.