It's not a problem. Remember that European showers usually have adjustable heads that also come completely off. I would put my standing toddler in the stall, take down the shower head and then adjust the water pressure and temperature. Then I'd simply hose off my kid, car wash style.

You also might be able to adjust the shower to the lowest setting for him. You do still have to stay.

Kid can also take a shower with mom or dad but have the other parent come get the kid so that the showering parent can finish in peace.

A blow up tub would also be a solution at a younger age.

P.S. Annoying that here in Europe, they're not big on no-slip mats. If you pack anything, put those decale suction thingies that you can paste on the shower floor. I think they're cheap enough you can toss them at the end of your vacation.