The OP being a WDW veteran, I'd tag a 4-day onto a WDW stay to get a flavour for cruising and whether it is for your family. If you enjoy a first shorter cruise, book a placeholder on-board and that way you can plan your next (maybe longer) with a discount and OBC.
The ships motion and how all are affected by (DW impacted significantly but manages with medication and our first cruise was like glass and was not till the 2nd she felt the effects), sea days confined to the ship (a mental thing for many), port options and being able to get through the Disney detail on water will affect whether DCL becomes a long term option for your group.
For us longer is better but I'd never discount a shorter cruise because that is what time-off or budget allow even now. We started 4-day, have done a number of shorter tied to a DVC resort (Vero Beach prior to a cruise our preference and/or WDW stay tagged on a well), a number of week long, a transatlantic and will be doing a Panama Canal next year getting in the max days possible, but still consider a 4-day a good taste tester for someone questioning their first cruise. One of our most memorable was a 4-day Vero Beach stay followed by a 4-day cruise followed by a 4-day WDW stay which had us feel literally we had 3 vacations in 1 as each was distinct in what it offered but was all still Disney.
We book ours now thru a TA but did not at the start (transfer from OBB made) for the minor additional OBC obtained, but it is they knowing what we like (room wise and orientation) and managing the opening day process that makes the TA worth it for us.
Prior to us cruising, I'd never have thought we'd enjoy as much as we do and what we try and do each cruise is less than the one prior. I know our first few cruises we over-planned, tried to do all possible and if we'd done a 7-day to start, we'd probably have been as tired as after a WDW busy stay and may not have taken to DCL as fast as we did.