The children are in bed and are asleep by the time you get in the shower? But then one of them screams. What have you found when you rush in to them? Has there ever been something so serious it couldn't wait until you were out of the shower when you expected to be out of the shower? Yes, we get anxious when one of our children screams. It's mother's instinct. But we can train ourselves to react rationally instead of emotionally when we know that they are in a safe environment. You need that time in the shower.
If you didn't respond would they keep on screaming? Often kids have dreams. They cry out. But still continue sleeping if not disturbed by our concern.
Your children are old enough to come get you if there is a problem. The 4 1/2 yo is old enough to respond if the 14 month old doesn't remember to go to you. I'd suggest to your son that when you're in the shower you'll come to him once you're thru showering. If he has an immediate need, and go over what an immediate need might be, he's to come to you. Reassure both of them that they are safe even when you're in the shower.
The question for me is why are they screaming? Do they need a night light? Does the sound of the shower awaken them? Do they frequently scream in the night. Are they able to get back to sleep on their own when they awaken? do the go to sleep on their own? During waking hours do you always run to them when they seem upset? Perhaps if you knew why they scream while you're in the shower you could correct that situation.
If you decide that the late night scream is not a signal of immediate need perhaps you could have the radio playing soft music and the door part way shut so that you wouldn't hear their cry. Or have soft music playing in their room to see if comforting sounds help them stay asleep. Turn if off when you're out of the shower, tho. Music all night prevents us from geeting deep sleep.
Our babies become children become teens become adults. Along the way they learn how to be independent as appropriate at each age. I think that at 4 1/2 with a 14 month old sib they can be independent long enough for you to have a shower.
Taking care of yourself first sounds selfish to some people but it is true that if you don't take care of yourself first eventually you won't be able to take care of your children. You perhaps can get by taking care of them but you won't be able to give them the same level of nurturing that you'll be able to provide if you're as calm and rested as you can be. You'll never be as calm and rested as you need to be but you can have that as your goal. Mom first, kids next goals are intertwined with each other. It's a flexible way of parenting. Sometimes Mom needs something and the kids can deal. Sometimes the kids need Mom's attention first and she can deal. What counts is a balance. Finding that balance is one of the most difficult tasks in parenting.