My wife and I took our kids (ages 6, 11, and 13) on the Carnival Fantasy and we loved it. Let me get my small complaints out of the way first:
Upon arrival I had to pay $105 to park. Really guys? You can't let me park for free while I spend a few thousand on your ship? That's unfortunately chincy. Just add it into the fee for crying out loud. I hate to be nickle and dimed.
The pool is small and over-crowded. I understand you only have so much room on a boat for a pool, but dang it was always packed and crowded.
M66 on the Main deck was small (duh) but clean and nice. Beds were comfortable and room felt nice and cool. Nice view of the ocean from our window. Pay extra for the window...it's worth it. Plenty of closet space. Nice hot shower.