Translation:Let me go!
Maybe the root words would help you understand the difference.
Lepas = free, leberated, detached, released
Biar = let, permit
Mari = Come here, please
Mari kita = Let's
"Biarkan anak-anak itu bermain." / "Let those children play." https://forum.duolingo.com/comment/28989050
"Biarkan" in this sentence is "to give children permission to play". You can't use "mari" in this context because it's not an invitation/offering. I'm still figuring out whether "lepaskan" and "biarkan" in this context are interchangeable or not.