It would probably be easier to have the kids share a cabin when they are less than 9-10 then after. John would know better about that then me. At Glen's age, (5), he couldn't care less about girls and I would not have a problem with them sleeping in the same area. You might get away with splitting the V berth like I do. It does give them some of their own space, but three cabins would be easier. The problem with the third cabin on the boat is that it takes away (a lot) from the rest of the boat.
For me personally, I really like it when you get into the upper 40's and low 50's for three cabins. I don't know why, but this also seems to be when you start seeing the over/under bunks too which would be awesome for kids. Problem, of course, is the cost of these boats.
Also, you are really planning a long way out. I hope you can keep to it, but we can't hardly plan out a few months much less many years. We instead elected to buy our boat early and live aboard. I am not saying that is what you should or should not do, but that is what we did and are (hopefully) going to be doing again shortly.
The benefit was that our kid are as comfortable on the boat as they are at a house. They know their way around and there is very little adjustment.
Mom holding Chase...
Chase watchin TV...
The negative of course is the space and it is harder living aboard on a boat then in a house. Many people simply cannot make it work. However, you are at an easy age to make it work should you decide to do so. I think infants especially are very adaptable to the boat. And as they grow up, instead of soccer, they find many other things to do that are more boat related, like fishing and taking out the dink.
But like I said, things are tight. It is not easy. But growing up with it allows them to be as comfortable on the boat as in a house (or, almost at least!).