Children play in the sand surrounded by a sand structure with a lot of window like openings.
The children playing in the sand all know each other.
As the amount of children isn't stated there could be between 2 and thousands at the beach. It's possibly that two might not even know each other and might just be playing because they're both kids.
Just because a bunch of children are playing in the same sand doesn't mean they all know each other.
Children play in the sand surrounded by a sand structure doesn't mean that they all know each other.