You can pretty much put water everywhere on a human body, as long as you make sure that they can still breath.

Legs and arms and the back etc are fairly trivial to clean with a showerhead. Skinfolds are where I would expect problems.

Btw, your system doesn't need to handle all corner cases of cleaning to be useful. It needs to not hurt anyone (in all cases); handle most common cases of cleaning; and ideally alert you when it can't clean some spots.

But even a system that can't alert you is still useful for a parent, because the parent can still look over the child afterwards.