I've heard a lot lately about designers recommending that people stay more neutral when they decorate their children's rooms and to stay away from the themed rooms. I say - why decide?
The pros of a themed room are they're adorable and make a statement. The cons are that they are quickly outgrown and can be expensive to replace.
The pros of a neutral room are that they last longer and grow with the child. The cons are that they can be a little boring and lack personality.
So, I say go for a happy medium. Skip the character wallpaper, but add some posters or wall appliques which are easy to change and fairly inexpensive. Purchase or build a large canvas and create your own themed wall with paintings, decopage, memorbilia, whatever suits your room ideas. A neutral comforter can be enhanced with lots of themed pillows and pillow shams. Buy lamps and replace the shades as the themes change.
Basically, go neutral on the big items that cost a lot, and then go crazy with the inexpensive little items that add charm and personality. This way you get the best of both worlds; a room that will grow without major changes, but enough changes that it reflects their personality and interests.