Take 'em! You talked about the pool -- that's only one of the many things on the
Disney cruise! At age three, your twins would be stuck in the Mickey Pool, which is often crowded at mid-day; we swam (alone) after dinner frequently. The Goofy pool (a family pool) is too deep for three-year olds, and the adult pool is, well, for adults.
At age three they're old enough to go to the kids' clubs, which are WONDERFUL. The Disney club for kids that age is decorated like a giant pirate ship, and the sky has tiny white lights making up constellations (the Tinkerbelle, the Captain Hook . . . ). They do crafts and face painting with the kids, they take them up to the sports deck, they have pajama parties -- they do not take them swimming. They stress cleanliness; when kids come in, they're directed to the sink immediately to wash their hands. Parents are given a pager so the counselors can call you if your child is fussy or wants to leave the club. Kids get a bracelet to wear during the whole cruise; it has your name and your muster area (spot you'd go in case of an emergency) -- they look at it and demand to know your codeword before you can take your child out of the kids' club -- no one else will take your child. The kids' club gives you some adult-time.
The characters pop up everywhere -- especially in the kids' clubs.
You can see movies in the afternoons and shows in the evenings (G-rated movies tend to be shown in the afternoons).
You can take them dancing in the family night club.
They have games like putt-putt and horse racing in the lobby often.
You'll be able to take them to great ports where you have a wide variety of activites from which to choose;
Castaway Cay (Disney's private island) is simply the best!
Believe me, you will not be bored!
Falling overboard is absolutely not a concern in the public areas. Everyone thinks that before they've been on a cruise; the reality is very different. The "main outdoor deck" where the pools, ping-pong tables, etc. are located is plexi-glass from the floor to the ceiling. If it weren't, the wind would be too strong. The other decks have high railings with plexiglass on the inside (no place to get a foothold). Plus, most of the public-area decks are "stepped up" like a cake -- if you managed to climb over the railing, you'd hit the deck below, not the ocean. My husband and I walked all over the ship LOOKING FOR a place where an overboard accident could occur; we found a place on deck four (near the lifeboats) where an adult who was trying could jump off the ship.
Private balconies in staterooms are a little different -- it would be possible for a child to climb on a deck chair and go overboard -- however, you don't have to get a balcony room.
Think about it: 100% of the people who have gone overboard, or who have disappeared from cruise ships have been adults. No children have EVER fallen overboard. Well, I did hear of one teen who jumped overboard while the ship was in port, swam around to the dock, and tried to reboard sopping wet with no identification; I don't think we need to consider him while talking about your three-year olds. Seriously, with all the incompetent parents (allowing children to run wild is a problem on cruise ships) and drunken parents on cruise ships, if it were easy to do, it would happen frequently. But children just don't go overboard.
Personal opinion: the Land and Sea thing doesn't appeal to me at all. It's too little time at Disney and too little time at sea. I love both vacations, but I don't want to mix them.
Having said that, I would suggest that you choose a 3-4 day cruise for your first time. While everyone I know personally has LOVED cruising, I know that some people get terribly seasick or just don't care for the venue. If you turn out to be one of these (unusual) people, it'd be better to be stuck out on the seas for 4 days rather than 7.