I find it really difficult to believe that a 4 or 9 year old would recognize a witch's circle that was supposedly seen in Sponge Bob, how would they know what this was unless they had already been exposed to this?
Sometimes children just need to veg out after school and watch silly things, we all grew up watching cartoons with somebody being thumped on the head, falling off a cliff, being splattered in the road by a car or something like that but we certainly didn't attach any dark meaning to it or if it were there it would have gone right over our heads. It was just funny!