I have 4 teenage children as well sharing the same bath. If you can figure it out how to make it look great please let me know. Seriously though I believe less is more. Find a great paint color because that will hugely impact the space. We have a sunshine orange color - supposed to be a very positive color for these young minds! What are you considering for tile, vanity, etc.?

Are all the kids OK with the walk in shower? Sounds good to me. You can put it right where the tub is and make it the same size or size it down to accommodate something else, storage? We did this with ours and the tile we chose for the shower makes the whole room shine. We chose a sea glass mosaic with pebble trim for a beachy look, but you can find colors to go with your family interests.

5x7 is really pretty standard for a bath - we see them all the time. A wall in shower will look great and open the space up - you'll be surprised. Keep things light and use a frameless shower glass panel and door.