I'm not a parent myself, but I frequently look after two siblings of close relatives. While the girl (3 years) does not have a problem with wiping hands in pants or getting in any other way ridiculously dirty, the boy (5 years) would rush to the bathroom if he detects a marker stroke on his arm and when he can't wash it off, he would remind you every hour that it is still there. He has been like that for at least the past 1.5 years and it isn't getting better. His mom told me that sometimes he would wash his hands every 10 minutes without a particular reason.
The two of them regularly stay over at my house for a few hours or days and his behaviour was never as bad as his mother told me, but I still need to assure him that it is fine if he has a stain on his shirt or something similar.
There was one situation that really stuck in my head:
We went on a walk, when we stopped at a bottomless fountain and I told them it is okay to play with in the water if they take their clothes and shoes off. When the boy got cold, he wanted to dress again, starting with his shoes. When I explained him that he couldn't fit in his pant legs with the shoes on, he started to get upset. He kept saying that he needs clean feet and he can only have them if he wears shoes. I didn't understand his whole fuss about being clean until he said: "But I have to be clean to go home with you!"
To be honest I was shocked that he thought he couldn't come home with me if he is dirty. I would have taken him home with me if he had been covered in mud from head to toe. So I took my shoes off and tossed them aside.
"So my feet are dirty now. I'm going home without washing them and you are coming too. I don't care if you are dirty or not, you are always welcome at my house. I love you, clean or dirty."
I'm not sure if he was more impressed by my lovely speech or the fact that I threw my shoes around. But after that we didn't have the same discussion again.
Even if I still think that my solution to this particular situation was quite good, I have the feeling that I should create more situations to show him that it is totally fine to be not perfect, that sometimes you get dirty during doing things and you don't always have the chance to take a bath immediately.
I know that the best would be to do this with him together and model the behaviour (walking barefoot, wiping hands on pants, rolling around on the lawn, taking a mud bath or drawing with skin compatible pens). I would also include his little sister (which really wouldn't need more encouragement), since I don't want her to feel left out and he would tell her about the stuff we do anyway.
While their parents won't have a problem if I return their children dirty or doing the above mentioned things, I can imagine that they teach this to others in the kindergarten ("Let us roll around in the garden, your mom can wash the clothes anyway!"), which may not be appreciated. Or being too much influenced and decide that they never have to bath again! Maybe there is also another good reason not to do it... or I'm overthinking things.
tl;dr: Should children be encouraged to get themselves or their clothes dirty (as long as it is not a health concern), while assuring them that this behaviour is fine? For example rolling around on a lawn or wiping hands on pants.