Have cruised on other lines with a seven month old and 2 1/2 yr old, and another time with a nine month old (husband with both times, though), and had no problems enjoying ourselves either on the ships or in the ports. With your dd's and dn's ages, I can't see there being a problem. Just have some more "stuff" to carry (bottles, diapers, wipes and a stroller). Otherwise, (then the "stuff"), it's no problem, it's a joy. I think you'll have a great time. Whose baby will this be a relatives, friends or your own? If it's your own, and you breastfeed, you'll have even less "stuff" to carry, and I'd also recommend that when you (or whoever)start the baby on solids, if it's before you travel, that you get a baby grinder for about $7-10 at a baby store (Babies R Us carried them when I got mine), and start the baby on ground table food. Easier then bringing along babyfood (which you have to supply on a
Disney cruise), and easier when it's time to transition to table food, not to mention very cost efficient. It was actually easier to just grind my own food (what ever I was eating) for the baby then having to lug jars. By the time we cruised with our then nine mo old, he was actually on table food, and I never brought out the grinder (we were in Russia, and I did not think that there would be baby food or formula readily available that he'd be accustomed to, and it worked out so easily!)
Good luck!