I just finished my daughter’s room (2.5) and she also LOVES Thinkerbell. My budget was very small so the furniture shopping was done at IKEA :). I didn’t want to overwhelm the room with all that Thinkerbell stuff so I decided just to use some of Thinkerbells colors (yellow like Think’s hair and green like her dress). I painted the room in yellow, I put 2 long green window panels, big green shaggy rung in the middle of the room, shaggy green bead throw (I made myself) and threw bunch of Thinker bell pillows that I got in different store all over her bad (they are all in different Thinker colors, purple, yellow, green and hot pink) then I finished the room with pink flower laps that go on the wall from IKEA (http://www.ikea.com/us/en/catalog/products/00097950).
I know that you have to combine 2 different styles in one room and Thinkerbell (light green, yellow, purple) colors don’t go so well with Thomas (blue, red, black), so it will be very hard to pick a sharing wall color (or if you don’t mind to paint the room in 2 different colors). I would suggest asking you daughter to pick a different princess as her team, like Cinderella or Snow White. Cinderella, Snow White and Thomas have one color in common that you could use for the wall….. BLUE (well different shades of blue but I will try to find something in between those shades….try not to get to dark of the blue). I would keep the window treatment very simple and match it to the wall color (BLUE). For the boys bad I would get a red bead throw with bunch of Thomas decorative pillows, fitting (Thomas or red) night lamp on the nightstand and red or Thomas image small rug in front of the bead and to complete everything I will frame a Thomas & Friends poster for his side of the room. For the girl (if you go with Snow White) I will also get red bead throw (you can also use yellow but having read on both beads will make the room consistent and not to BUSY), then same as the boy’s side: bunch of Princess/Snow White pillows with a fitting lamp, rug in front of the bad and a poster on her side of the room.
That is all I had for right now to suggest…..hope I helped a little or at least got you brain storming with ideas and possibilities :)
Oh, here is an image of Think (just in case you still want to go with her ) that could help you get inspired and help you work out the whole blue issue (the napkin has a little midnight blue going in the background): http://www.birthdaydirect.com/images/tinkerbell-flowers-l.... You can also use this napkin on the wall and frame it.