Do you own or rent? I ask, because the first thing I would do is paint. Maybe green; something not stereotypically a "boy" or "girl" color if that is likely to provoke disagreements. Because the space is so small (especially for the number of kids), you don't have a lot of space for art or "stuff." Adding a runner on the floor would be one opportunity to add color and interest, as well as being functional.

Depending on the ages of the kids, you might think about a full length mirror on one of the wardrobe doors. It would give an extra place for hair brushing and primping (I'm assuming since you are short on bedrooms you are probably also short on bathrooms).