Personally, I think a 3 or 4 day DCL cruise is perfect for new cruisers and tacking that on to a WDW vacation is awesome.
We did the 4 day Bahama cruise June 2014 and added 3 nights at Wilderness Lodge AFTER the cruise to surprise our kids. lol
It would work out great either way though since getting on that DCL bus to go to the port before your cruise is pretty exciting too.
I would suggest setting your dates for the hotel stay first and then looking for the Bahamas 3 or 4 night cruise to work around that.
Just a note though, early December rates are priced less than those dated after the 18th due to kids being out of school etc. In fact the prices have been known to jump from 2500.00 to upper 5,000.00 range closer to Christmas. Of course pricing is based on room type and location, but with a family of 5 adults, you would either want to connecting rooms or 1 Family suite.
My kids are 12 and 13 but are adult sized in height, so we get a ocenview view with Verandah which has 246 sq ft of space including the balcony.
At some point, we will have to consider a family oceanview though just to have more space. lol (Unless they stop growing that is)
The itineries for 2016 are out and I recently booked one for a client for September 2016 and got a good rate for them, but as soon as the 2017 dates drop, you might want to check those out because prices do increase after the inital announcements.
Check out the nooks and crannies of the DCL website and click on everything! There is so much information and so many FAQ's and answers that will help you out too.
Have a great time planning! It is my favorite part.