Many companies online and offline offer personalized prints that are great for kids' rooms. One fun place where you can find very unique options are fairs, where vendors often have caricatures, silhouettes, and other “fast art” that highlights your child's name and/or interests on offer.
There are also places online that will add your child's name to a pre-existing print and frame it for you. No matter what you choose, personalized wall art is fun and special and unique to your child.
If you are working with a theme, it's possible that the company that makes or sells the items you have used may also offer wall art that goes with the bedding or other accessories you have. One or two pieces like this should suffice, as you want the room to look unified, not cookie-cutter.