I've slept in all types of cabins. The curtains they put in the cabins totally block out the sun. Your baby should be fine napping in the afternoon. I will tell you that the inside cabins are pitch dark all the time and that can be very very nice. You will need some kind of night light in there so you don't have to keep the cabin lights on all the time. The inside cabins are smaller so you will risk the chance of waking the baby.
Life is too short to let the ship of your dreams sail without you.
Carnival Destiny Feb. 2006
Carnival Fascination Feb. 2007
Royal Caribbean Grandeur of the Seas Feb. 2008 The DTW & MsBJ tour
Carnival Valor MsBJ and Dina Feb. 2009
Carnival Glory MsBJ and Dina Feb. 2010
Carnival Dream MsBJ and Dina Feb. 2011