DD has spent the summer doing cartwheels on the lawn etc and by the appearance of her trousers and leggings, she has landed on her knees more times than she has landed on her feet 
Several of her trousers have got grubby knees and ground in mud stains. I have washed with biological powder and pre-treated the stains but they're still there. I seem to be always nagging her about the mess of her knees and yet I feel bad because I'd certainly prefer her to be outside running around doing energetic things than sitting indoors keeping her clothes spotless.
Nevertheless, I want them to look cleaner! Are there any tips that I need to know about?