I get duo lingo is teaching us a difference between bermain and bermain-main, but i dont see the difference in the English language between they are playing and they are playing around.
You can play a rigidly structured game or sport but to play around is to pay little attention to any structure or rules and just do what feels like fun.
There is no "di sekitar" so it shouldn't have "around".
bermain-main = to play around
di pantai = at/on the beach
bermain-main di pantai = to play around at/on the beach
di sekitar pantai = around the beach
sekitar = sekeliling
bermain-main di sekitar pantai = to play around around the beach
'play around' has a sexual connotation in the beach. So does main-main sometimes but I would say it's less than play-around. I think just 'they play on the beach' should be accepted.
It can have a sexual connotation, but far from always. I wouldn't hesitate to say that children, dogs etc are playing around
In my opinion "play around" is perfectly acceptable, and was the first translation my mind went to.