Note the rustic pine bunk bed and hatcheted stump ceiling fan. See too the mural painted on the wall.
In a less elaborate version, I've seen pine furniture with wainscoting made from rough pine boards (varnished naturally or lightly stained) with dark green painted walls. Dark green, blue, and red plaid flannel valances and bedding complete the theme.
In either case, today's kids' rooms demand more than Tiger Beat or sports posters on the wall. [smile]