Ruth, most cruise lines won't have prices available until 18-12 months before the cruise. For example, in May 2008 Royal Caribbean will come out with prices for summer 2009. Even the ships sailing Alaska in 2010 won't be confirmed until that time. With all the cruise lines coming out with new ships every year, ships change cruising areas and itineraries all the time.
Children are not allowed to be booked into rooms without an adult. Some cruise lines do allow children to be in a connecting cabin. Usually, each cabin has to have at least one person who is 21 or older. Some cruise lines say 25 or older. There are some ways to get around this, but remember, it's the safety and comfort of the children, and everyone really, that leads to rules like that.