It was my understanding that all cruises offer Aqua Babies regardless of numbers of kids on board. RCI's youth program is offered all year long on all ships so your four year old will have somewhere to go.
I did transatlantic when my daughter was about 11 years old and she loved the slow pace. The only problem with your 9 month old is that all ships have signs by the pools that those in diapers are not allowed to swim in the pool. Thus, there won't be too much to do with your little one. AquaBabies is good but only 45 minutes per day.
Let me know what you decide.
Luisa Frey Gaynor
Family Cruise Editor