Install a ceiling mounted curtain rod meant for claw foot tubs.

^ agree, this is a good solution - weights at the hem of the curtain keep it from flying around and sticking to skin

Neither of those keep the wall dry the tile only goes up 1/2 the height it needs to be.

It’s a 360 degree wrap-around curtain, so it would go in front of tile. Great idea!

curtain will work as long as person taking shower doesn’t wave wand too high. Keep a towel handy to wipe down the walls after every shower.

FRP (Fiberglass Reinforced Plastic) is cheap, comes in sheets, and would be easily installed.

We are talking a 6 yr old boy and water good luck with keeping that wall dry . I think best idea is tat sheep of plastic actually adhered to the wall and all edges sealed This will last until you can do it right and he grows up a bit.

Joseph is correct. Temp fix FRP panels with panel adhsive and the associated trim pieces would be the easiest.

Would agree that FRP glued to the drywall would be a cheap, simple stop gap. That would allow continued use of the soap dish and current method of product storage.

Note bathtubs are almost always tiled over drywall, which is not allowed as a backer with a shower surround, as water will get through the grout joints, so keep on eye on the caulk, mold, and softness of the wall. Life always gets in the way of "stop gaps" and they become permanent.