Childrens' Garden... Interesting.
Now that is a case of just little enough education and over-thinking a problem. (As opposed to most of the signs I see with errors, where the writers clearly have no education about apostrophes and just guess: "fresh tomato's on sale here," "keep door 'closed' at all times")
Here, the sign writer knew, presumably, about the "apostrophe at the end of a plural noun" rule - it's the rhino's enclosure if there's one inside, but it's the rhinos' enclosure if two or more live there. But children is already plural! But it's for all the children! What to do! Lots of childrens own it! Childrens' garden?
Amusing.