I have 6.5 and 5 yr old girls and they have gone to DL almost every month since birth. They have also been to WDW at ages 5 month (the oldest), 2 and 8 mos, and at 5 and 3. We are headed back there again in June, because of my involvement with the DIS Unplugged (this website's own podcast).
That being said - I would do the one night pre-cruise, but definitely save a vacation for older - at least 8. (This is a big change from my original thoughts on the matter.)
Why? A couple of reasons.
We were stuck on the fine line of trying to include more than the kids or I could handle. It's like herding cats. You see a lot of frazzled families in WDW that are trying too hard, rushing from one place to another. I felt that and had to scale back my plans. We even skipped a theme park day because of it. Many families don't feel like they have that option.
Distractions. Kids that age are of the "ooh shiny" genre. It will take you much longer to do an activity than you expect, or that any commando planning will allow for. You, however, know that from
Disneyland. It's better and more satisfying to go with the flow and follow their lead. That is the way to feel like you have really gotten the most "planning bang" for your buck. Otherwise, you run the risk of being disappointed or needing a vacation from your vacation (and possibly your child).
Dining. We would often make dinner reservations (ADRs) and have to cancel them because we needed to feed the kids sooner or later than the reservation called for.
Walking. While they did well for short jaunts, we had a double stroller to rely on. The oldest (at 5) was in it quite a bit, due to the sheer volume of walking - they haven't used a stroller at DL for a few years now.
I'll be interested to see how mine do this year, now that they are older. I think we still plan on taking it slow and easy!