You said it "the parents seems fine with everything" so take a deep breath and relax. The parents are happy with you and them sending a thicker cream is just them realizing the other cream doesn't work well. Before nap I slather on the cream as most of my kids will poop during nap.

As women we look for non verbal cues from people and may be oversensitive to a cranky look or demeanor from a parent, but it could just be they are in a lousy mood that has nothing to do with us.

I have a mother who is constantly questioning me and has a "I know it all better" attitude. At first it really bothered me, now I just shake my head at her and realize she is insecure and trys to make herself feel superior to feel better about herself. I ignore most of her remarks now.