oh.. another thought..

I think it'd help if you talk and fuss about the new room leading up too. Take them on a shopping trip for new doona covers and decorations for the room (let them choose) and then get them involved in putting it all together. Then you could have a countdown to the 'big-move' and talk about how great it is going to be having their own room. You could also do some playtime in there each day so they start using and liking the space, etc.