Install a floor covering that mimics the look of a green soccer field. There are a variety of ways to accomplish this, such as laying down an inexpensive indoor-outdoor carpet over the existing floor or carpet or painting a subfloor in usable condition with green paint to emulate the look of grass. If none of these options work, consider a green area rug. With any option you choose, finish the look by using spray paint to make field lines indicating the half-way line, the penalty line, the goal areas, the center spot and the corners.
Place furniture in the room that mimics the look of goal boxes. Purchase specialty tents that fit over a twin bed to make it look like a goal box or a play-sized soccer goal box.
Paint a mural on your wall, apply a vinyl wall sticker or hang a large wall art to emulate the look of a soccer stadium. Draw the stadium freehand on the wall, and fill in the drawing with paint. If freehand drawing and painting are not your strong suits, apply a vinyl wall graphic that looks like a soccer stadium, or enlarge a photograph of a stadium to hang on the wall.
Accessorize with soccer-themed elements you would see in a stadium. Place banners on the wall indicating the soccer team's name, or place a large beanbag chair that looks like a soccer ball in the room. Place a metal bin for soccer ball storage in an out-of-the-way area. Personalize your room by hanging a soccer jersey with your child's name on the back of the wall.