I'm taking my six year old on our next cruise, it will be her first. I think she is finally old enough to enjoy it and the cruise lines have finally come around to proving family options. I am of course going on a more family centered short length cruise 4 days during the weekend instead of a weekend party cruise. I think this will be long enough for her to experience everything and short enough that she doesn't get overstimulated. That being said it is totally off the subject of what you are asking.
I too believe, and I too am a C&A member, that RCCL is concentrating short-sightedly on profits in a lot of areas. This just appears to be another example. While I would not take an infant on a cruise, I can see why someone might want to especially in the situation you have outlined where its a family vacation. You shouldn't have to pay full price. There should be a reduced fee of some kind. I'll send a note to the addresses above in support of you if it will help, but I would agree with one other poster and ask the family if you can find a cruise at the same time, same length for same or less money would they consider moving their trip to a more hospitable cruise line.
I've done group cruises before and have had the necessity to change lines after the inital deposit was made due to a lines inability to accomodate our group as initially we believed they could or would.