They usually offer some sort of stateroom credit. When I was just on the Wonder last week I noticed they were offering a $200 stateroom credit if you booked a 7 day cruise.
You'll also get the early booking discount if you are booking a cruise that is farther out.
You don't need ALL of the money to rebook onboard. You'll just need deposits for each person (about $200 per person for a 7 day, but don't quote me on that)
I've heard of some who will rebook for only the ADULTS in the room and then will add the children later---that way you only have to pay for the adults deposits onboard. Just make sure that you get a room that will hold all of your potential travellers. AND....cruises CAN fill up based on the number of kids onboard (so they don't overwhelm the kids' clubs) so make sure you don't wait TOO long to add the kids.
You can also book a cruise to get the stateroom credit but then change the dates or itineraries and the credit will follow you.
HTH. I am not a cruise expert (yet!) so if there are any weird things in here I hope someone clears them up!