My friend was looking for inspiration for her twins (one boy, one girl) bedroom. Eventually they will be moving them out of their cribs into big kid beds, so she was trying to come up with a theme for the room that would accommodate both children. I have to say, it's not an easy task!
My suggestion was to go neutral or soft blue on the walls which ties in great with pinks, greens and yellow.
Of all the images on Pinterest, my favorite is this grey, green and blue room. It's still juvenile enough for little ones, but would transition nicely as the kids get bigger.