Hi! Here's what I found on the Carnival web site:
"Toddlers do not have to be potty-trained in order to be part of Camp Carnival; parents simply need to provide the youth staff with diapers and toiletries for their child."
The pools are not heated as far as I know.
We haven't sailed the Fascination, but I'm sure someone else will be along to answer your other questions
Have a wonderful cruise!