This can be challenging and fun. Let the child be a part of this because they can come up with a zillion ideas. First work out a theme, color and then go on to the matching bedding and so on. If you can't paint the walls then try and work with some wall art or divert attention with some rugs or pillows that can distract the attention from the walls. Stenciling and free hand art can also work wonders when doing up a child's room.
Try and get wall art from a local craft store or a retail store where it is cheap and still looks good. Make sure that all the items that you purchased are safe and can in no way injure or hurt the child. Make sure that the child is as much a part of this project since it is their room and it can be fun to share their ideas and work with them.
Since it is their room and they have to use it, it makes more sense to make sure that they like what is being done there. As this can be such a fun project you might have to keep tabs on the budget and not go overboard since there are so many options while doing up the child's room.