Just a thought, could your have 2 bars to hold the shower head in 2 different locations. So you could have it on one or the other bars to suit your needs without having 2 heads; keeping it near the entry for one use and moving it to the other bar for typical shower use.

Or you could just have two setups, one near the door and another head in the typical place. Each with its own on/off temp controller.

Thank you. We definitely want two shower heads. I'm not sure where to put them. On the long walls across from each other?

Well, one shower head you want to have near the door, right, for your daughter, and on a bar? The other one could be down the way a bit on the same "north" wall in your drawing. It could be a hand held too.

Or get this setup :)
It is Noopd's shower and is about the 5th post with the picture of a shower setup that I loved so much that I got it too. I haven't installed it yet. He has a somewhat small hand held. I will have a bar and a larger faced shower head for my hand held. I can't wait to get under that dual function rain head.

Here is a link that might be useful: Noopd's shower setup

I'd recommend ditching the built-in bench. Any time there is a special needs type of shower it's usually better to have a movable stool or bench. You could even have both. A moveable bench-for-two where your built-in bench is on the plan. Then a stool in the southwest corner of the shower, immediately to the left as you enter. Wood is always warmer on the skin than tile.

Example, you could pull the stool over by the door for you child to sit upon while you stand at the shower entry using the handheld.

I'd put your handheld on a vertical bar at the midpoint of the north wall. I'd have the valve that controls that head to the right of it, on the right side of the north wall, just inside the entry. Near that valve, I'd also add a wall bracket for the handheld.

When the handheld is used in typical fashion, it can remain on the vertical bar. When you need it to bathe your child, you can hang it on the wall bracket by the door when needed. A "third hand" so to speak. It can also be at a convenient height as needed. Wall brackets are fairly inexpensive. You could even put two in there, one low, another up higher.

Put a long hose on the handheld. Consider an 8' hose. It'll be easier to maneuver.

I'd mount the fixed head midpoint on the south wall. If you're going with separate supply valves for the two heads, which I recommend you do, I'd also have the valve for the fixed head by the door near the handheld valve.

Your shower is large enough for two bathers, so two independent supply valves allows two different shower temperatures.

Thanks! This is exactly the kind of information I am looking for. Just for clarification though, and I'm not sure it matters, but my special needs child isn't physically handicapped. He needs assistance because of his cognitive impairment.
We are planning on separate valves and I'll look into extra wall brackets too .