I think sending kids home dirty is absolutely fine. I do not try to keep them clean. I clean them up before going home and change them if need be. I don't think that is the issue here. Isn't it more about damaging snow gear and potentially hurting their knees in the warmer months?? I understand a parents concern with this. I think if an activity is going to damage clothing then other arrangements should be made. I think if an activity is going to hurt a child's knees, then thought needs to be given how to make this more comfortable, like placing foam mats down or something like that.

Old clothing or not, activities causing undue wear and tear isn't the best choice IMO. Wearing old clothing because there are paints and pens, dirt in a garden and puddles to jump in is unavoidable and part of play, but I think there are ways around avoiding damaging clothing and hurting knees on a play surface when there are alternatives.

As for your daughter, I don't think it really matters. It is a parents prerogative to choose whatever they like for their child and if she is scraping knees and getting dirty that is in your hands and a choice you make. This parent should also be afforded this. But that is on them. Its not that there is anything wrong with what they are doing or that these things don't happen during childhood, but just because you allow your daughter to do it doesn't justify the same thing happening to her kid as being okay for her. It's not right or wrong, just that this isn't okay with her when it is so clearly avoidable. I wouldn't worry too much about what she said. It sounds like she wanted to bring it up with you but maybe should have been more direct so you weren't left wondering exactly what her concern was. Now you know this kind of thing is a problem to mum, perhaps consider how you can avoid the scuffing of knees while playing on concrete. If I picked up my child and they had scuffed their knees by falling etc I'd think its okay, just childhood accident. But the play area and surface being the cause of the scuffing, versus an accident, well I'd be asking that there be foam mats put down or a softer play area provided to prevent unnecessary hurts and holes in pant knees.