We are taking our 4 year old son on a cruise in a couple of months (very excited). I wouldn't pay double for a room with a balcony. From what I've been told, you spend very little time in your room. We spent a little extra for a window, but I don't even think that it opens. I would imagine that the recirculated air isn't much different than being on an airplane - not the healthiest in the world but I wouldn't waste much time worrying about it. My advice, take Airborne faithfully while on your cruise (I will doing that with my son). We are taking his vest and his nebulizer equipment to do his treatments. I wouldn't mind leaving the vest at home and doing clapping instead, but I think he will prefer using the vest. It will be a pain to haul it, but once there I think we will be glad that we have it.
Have fun on your cruise!
Jena