Cass, did you look into one of those Combi double-folding strollers? Don't forget, Kira will almost be 3 and won't need the really big one anymore. Might be able to cut back there.
As for the car seat, what's the rental rate for that from the rental car company? Is it $8 a day? I've never done it and I don't recommend it (don't know what condition it's in), but it would be an option. Can't remember when DS was in his booster types seat. That stored well in the closet on the ship. Kira might be a little too young/small for that still.
Doesn't
DCL have a holding area in the terminal? If not, they should get one.
Are you staying at the same hotel on both ends of the trip? If it is WDW, hey may hold it in the luggage area for you. Worth a try.
Good luck.