D.B.
I agree she could be jealous of your competence in areas where she doesn't feel very skilled. There could be a privacy issue with the shower - some people are just nervous that way, either from their own experience or because they are nervous that some child predator will snatch their child. It may be that she feels she needs to supervise the kids, either because she does it at home and can't believe they're getting more independent, or because she DOESN'T do it at home but feels the need to measure up while you are observing her. What she's missing is the kids' joy of using an outside shower - one of the stories kids always tell from their growing up years. It's better if everyone just says how fun the shower is. I'd put a sign up in the bathroom about putting the liner inside the shower, or just keep it inside at all times if the sign will offend. Sometimes you can find fun, whimsical signs at vacation area gift shops that list the house rules in a funny way.
When you say you buy their wardrobe, does that mean you are paying for it or that you are taking them out and doing the purchasing? The latter may mean she feels you are usurping her motherly role.
Are there ways you can compliment her? "I don't know how you do it - I'm so jealous of your professional abilities." Did you work outside the home when your kids were little? Do you understand the guilt and the multitasking she is already doing? Does she KNOW you understand? Do you ask her about her job, or do you focus exclusively on the children? When you say you are generous to her and your son, does that mean you are giving them checks? Could you consider giving her a "spoiling" day at a spa, or a series of gift certificates she can use use to restore herself from her stressful life?
Since the summer is over I'd let the shower issue go, and also talk to your son about how you can get closer to her and see how you may have inadvertently offended her. Perhaps she feels she will never be able to provide a vacation home or financial support for her future grandchildren, because they are just making ends meet on 2 salaries as it is. She may feel she'll never be the cook/cleaner that you are. I wouldn't write a note about this issue but you might write a note to both of them and say how much you enjoy having their company.