We don't use that playground that much these days, but I do remember telling my children not to run around the "wee tree"!!!
I am not saying that this is acceptable but in the 10 years of using that playground (and I spent A LOT of time there !!!) I think I might have seen 2 / 3 kids maximum weeing by that tree. I really do not think children regularly use the tree as a "urinal" as some of the posters say above.
I have never seen anyone weeing by the bins - now that is even more disgusting and I really do hope people don't really let their kids do that ??
On a separate but related note - my boys tend to tell me they need the loo when they are literally about to wet themselves with no warning at all !!! So I have to admit that we have occasionally had to use a hidden bush on the common..