The rustic design style can really be a very fun element for a kids room. It’s gender neutral enough that you can even use it in a playroom. You can bring in a lot of wood furniture which can really last a kid for a very long time and transition into more sophisticated themes later on.
As far as the furniture goes it should be quite rustic. In this case, you can go for a pine or oak finish. You can also make this a lot more sophisticated by going with an armoire instead of a typical dresser. Be aware that this will be the focal point of your entire room because it is so large and it will also be one of the more expensive pieces that you can come across. Look for a smaller version because this is one of the rooms where scale is the most important. It can have a pine or a distressed look to it as well. In fact, this is one of the few ways you could make pink or purple make sense with this theme. It really still works into this design style just by the doing some simple faux finishes.
You can also really work in bunk beds in this kind of design style although you want to skip a lot of the metal versions even though they will be some of the least expensive options for you to buy. In this case, you may instead really just want to go for more of a rustic look. Usually this would seem very dated but you can still work it in with this kind of design style.
You can have a lot of fun with fabrics in this room that will match wallpaper borders or fitted wardrobes. In fact you could even get a cow print to use on your draperies. Really going over the top in these areas makes more sense because they are fairly easy to replace.