The Original Sleeping Beauty(Disney Version) had three fairies, yes but we don’t that the Aurora in Once is that Sleeping Beauty because of her comment that Maleficent went after her mother. In the original tale, the original Sleeping Beauty pricks her finger on the spinning wheel, falls asleep and is then awoken by TLK. Then the prince and she get married and have two children, one of which is named Aurora. So we don’t know if the Aurora had the fairies or really what her story is. It’s worth noting that, I believe, in the original tale–non Disney–there were 7 good fairies.
And as to Henry and the Playground, he still went there. In fact he buried the book there when they thought times were dangerous, after Regina killed Graham. So it’s very possible that August saw Henry or Emma from a distance either talking there or visiting there and surmised that the book would be hidden there. It was their secret place after all, just like the book is their secret.