I think you can buy those light switch stickers. Saw them on sale at kwerkee some time back. My friend's kids room have those as well. I think it's cute.. maybe instead of the normal lights you can use the kids light from Philips or even those at IKEA. Not sure carpet will be a good idea cos of static and it attracts allergen especially if your kids have sensitive nose it's not really advisable. But yeah, pads or rubber mats will be nice.
Definitely make it light, and creative. I think it helps with the kid's growth because that is where he spends most of his childhood in. So make the most of that opportunity and fill it with positive energy and creativity.
I suggest that you go for wallpapers instead of paints for the walls.. So it would be easier to remove and change when our child outgrows the design of the walls. As of now, try cartoon characters as your kid's bedroom wallpaper.
Other than the decorations itself, I think it's important also to consider the safety of the furniture being in the room. Sharp edges and corners is a no-no.