Maybe you have never worked in child care??
I completely understand what the OP is saying.
We asked over and over and over that the kids be sent in play clothes. We painted, we played with clay, dirt, mud whatever, we went outside and we played in the sand. WE GOT DIRTY! I had a little girl one year that was dressed to a "t" every day. Really cute, every thing "matchy-matchy"; and the little girl was miserable. She cried when we painted and wouldn't play outside (just stood in the center of the playground), all because Mom thought it was more important to look nice than it was to be able to be a kid.
Also, I am assuming all parents want their child's child care/preschool program to be clean. In child care that means cleaning with bleach. Clean with bleach enough, you get bleached clothing. Very frustrating when you are having to dress to a dress code. We never allowed inappropriate dress by our employees, but we wanted them comfortable and ready to "get down and get dirty" so to speak. I told many a preschool teacher, "if you cannot sit down on the wet, dirty ground and play with the kids; you are overdressed".
We had "wet and wild days" every summer. The kids would play under the water hose, play with water guns and bubbles. There were times that I sat right down on the ground and shot the water guns and blew bubbles. The kids loved it and I got lots and lots of hugs!! (the greatest pay for any child care professional)
OP, maybe your employer is trying to avoid t-shirts with inappropriate ads or sayings on them? If there really is a misunderstanding, I would simply ask for clarification. And I agree with you that short skirts and dresses that "show it all" are a lot more inappropriate than most t-shirts.