rheny,
Welcome back to the hobby!!. Let me see if I can answer some of your questions.
DCC Ready- Locomotive is wired to accept a DCC card and be run on a DCC layout. It currently has a jumper installed that will allow it to run in DC mode.
Track- DCC, DC doesn't matter as long as the track has been made in the last 10 years. (Stuff from the 1970s would not work.) For DCC the digital signal from the controller is sent to the engines to the DCC card in the engine. There is a lot of personal preferences on current track to use, I personally use Atlas code 83.
DC engines on a DCC track, they can be run on a DCC wired layout, but is not much fun. IMO.
Cost. Direct Current is cheap to set up and run.
DCC costs a bit, but you get more personal handling of the operations of your engines.
You mentioned you have children, not knowning there ages, you might not want them playing with a engine (DCC outfitted) that could be worth over 200 bucks. DC is cheap, simple to use and simple for kids to use, (I could be wrong, I have no children yet). If you plan on going the DC route I have two railpower 3170 controllers I am selling/trading. At this point I would give them to you.
--Cid