I was going to add this to the "over 35s" thread from a couple of weeks ago, but it's kind of fizzled out.
But, I have found a dress which is cotton, properly knee length, has short sleeves, not bizarrely patterned and not body con clingy. I've been wearing it for casual Friday at work today, now wearing it with Birkies to take DS swimming.
White Stuff Dress
I didn't pay full price for it though, but it ticks so many boxes, I think I would.