I will be in the minority when I say this, but I would stick with your current plan. A 3-day cruise is great to assess if you will like cruising. If you do, you have the excitement of "it left us wanting more." You can also use your short cruise to snag the 10% off booking discount to book your future 7-day cruise. Almost everyone I have ever talked to in my circle of friends over the years has enjoyed cruising, but I have had those one or two people who said it just wasn't for them. So if you do have some doubts, I think it's better to start small and work your way up to a longer cruise, rather than to start big and wish you had done a shorter visit. (I personally think it's impossible to have a bad time on a
DCL cruise, but you have to realize that this board is full of DCL disciples and fanatics, so there aren't many of us on here who say we don't like DCL, haha!)
As far as your toddler, you'll have to reserve times in the nursery for her, and they go quick. She also cannot go in the pool. (The pools aren't that great anyway, more like knee-deep wading pools on Dream and a big 3-foot deep bathtub on Magic.) The ships do have excellent splash pads for her age. I think cruising is great for toddlers because they will set up your cabin with all the needs (pack and play, bed rail, diaper genie) and you can go to your stateroom to nap when the need arises. Plus it is a lot cheaper to cruise when they are 2; when they turn 3, the price goes up.
Finally, I will end by saying that we like Disney World, but we LOVE, ADORE, and WISTFULLY FANTASIZE over DCL. It is so much more relaxing than the parks. You don't get nickeled and dimed to death like the parks.