Its hard being in a shower stall, all together... unless it is a shower over a tub combo set up.
JUST a shower stall, can be cramped unless it is a bigger sized shower.
I have a 3 almost 4 year old... he does not like showers... except if we are IN there with him, showering TOO. And it gets cramped in there... and it is slippery... even if we have those slip proof stickers on the shower floor. My daughter is 7... and since 6 years old, she likes showers and can do it all by herself.
With a 16 month old... they can be in a "Eurotub".... my kids were in there until about 2 years old and they are big kids. But you could use the toddler tub IN the shower stall, for your younger child.
All the best,