Good advice all around. Also, make sure all outlets are GFI protected.
If I were doing it, I would run a wire down off the doorbell circuit and install a ringer so they can hear it if someone comes to the door (hint, it'll probably be for them anyway).
Also, I would put a push-botton switch on the top of the stairs, wired to a small bomb or loud bell so you can get them up in the morning without having to run downstairs 14 million times. Maybe even a PA system to keep yelling "hey, it's time to get up".
I would also consider a second phone wire, just in case. Or at least have a 4 wire phone line going down there.
Another post suggested a hard-wired smoke detector in each room. That a great idea because the kids sleep so deeply. Get the kind that have both type of sensors -- radioactive and photoelectric -- because they detect things differently. Also a hard-wired carbon monoxide detector. Finally, make sure that you have a hard-wired detector near YOU so you can hear it if there are any problems.
Most importantly, have the basement checked for Radon before you have them living there.
Finally, do you have heat down there or do you need a bit of electric heat? And what about a circuit for a microwave?