Kids learn through play.
Children learn differently than adults. Their young developing brains and bodies are primed to wander and explore. They learn how to coordinate their bodies and minds by running, jumping, tumbling, laughing, and playing games with their friends.
If this is how children learn, why would we expect jiu-jitsu class to be any different?
Now, let's be honest. Playing just to play isn't enough to create an adequate learning environment. To avoid complete chaos, play must be directed and focused. Play must have a purpose.