Very well explained, but unfortunately that is how it works. Fares can and do go up overnight. Sorry about that.
I'm still undecided on the Early Saver. It's great for those that use it correctly, but so many times people don't understand that they are 'locked-in' and can't make changes.
There are a lot of families that do exactly what your doing and you will have a good cruise. Decorate your cabin doors, have fun, and make it a family experience.