These actually work pretty good. Doesn't sound like what you're thinking about but it is an option. Kids never mind getting into this to clean up and the extra space you can put stool in to sit on or hold cloths to change in a dry area. The water drains out the shower side. There's tons of these type on the internet. The thing with this sort is you can move it around your camp area where it works best. They fold up easily into a small space. Those heated black plastic tanks work well for shower water.
Have you seen the
wall ups? Sort of pricey but may work or something like that.
How much privacy are you wanting and how much space and weight are you willing to travel with to provide this?