I let my kids pee on trees too, but I always make them go away from the main play area, still close enough for me to see them, but not so close that anyone is so likely to run through it. I have been actually considering buying a fold-up potty for this though. I read a great idea (through CCPRN newsletter) of putting a diaper or sanitary napkin in a bag inside the potty to absorb the pee and then you can throw the whole thing in the garbage (not exactly environemental though!).