Don't worry, we were on the Sovereign two years ago and during the days its very casual. My kids dressed for formal night and wore nice clothes for the rest of the dinners (I put my foot down on that one too!). The remainder of the cruise I let them dress like they wanted. They spent about 90% of the time in swimwear. Make sure you get pictures on formal night. I found its the one time I have the whole family all dressed up in one place!