We've been researching this and Royal Caribbean do offer a transfer on a coach to the airport in Venice. It's pricey at £39pp each way (we were only looking at the return leg), there is no price drop for children. There are multiple times available.
You also need to have flight details for airport, we don't fly out until the following morning and whatever system they are using seems to be struggling with this. Might try and find a flight leaving that afternoon and give them those details instead and see if that makes it easier to book. You have to ring to book, you cannot do so online or over email (not a clue why!)
The other option we looked at was the train but with small children + bags we're probably going to take the hit and book with RC.
Must admit to feeling a bit tricked that they still use Venice in the port name and it's actually not Venice and the extra hassle and costs that now entails.