You can use one valve with a diverter and two shower heads, but it's going to still run about the same if you use two valves.
A diverter does wind up taking less space. Some valves incorporate the diverter on the same trim.
The bigger problem, if you lower the shower head for her, is having the taller kids knock into it. You may want a slightly longer arm for the high shower head.
Most corner showers are very small. A typical NeoAngle pan is 38" on the the wings, which works well setting the shower arm at 14.5" from the corner on the rough-in. Centering doesn't work well with a neo angle cutting back on the space behind.
When we install tubs, we center at 14.5" too.
Is there room for two basins in the room? One low and one higher?