We have 2 bathrooms in our house...this bathroom is the one our kids use, as well as the one that guests use. That is a bit of an odd combo, but I don't think it's unusual....I'm sure plenty of 2 bathroom families do the same. The challenge is that it has to be functional for the kids, yet look decent enough to make guests feel comfortable.
A nice shower curtain does great things for a bathroom. High impact, and not expensive. Our curtain came from Target. Adding in some decorative mirrors works well for when you have a bathroom without a window, that needs some extra light to reflect around. A mirrored bathroom tray further reflects some light, and provides a place to keep the scented oil diffuser as well as other bits and bobs. I got an acrylic counter top organizer (huge fan of these) to wrangle in all of the little hair accessories, brushes, a comb, and other frequently used items. Finally....a sizable plant will do the trick for making the space look not TOO kiddish. Plus, without a window as so many metal fixtures in the bathroom...I think adding a plant brings balance. I know nothing about feng shui, but I feel pretty zen in here despite it not being my favorite bathroom on the planet.