Firebase for Android, How can I loop through a child (for each child = x do y)

The easiest way is with a ValueEventListener.

    FirebaseDatabase.getInstance().getReference().child("users")
            .addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                        User user = snapshot.getValue(User.class);
                        System.out.println(user.email);
                    }
                }
                @Override
                public void onCancelled(DatabaseError databaseError) {
                }
            });

The User class can be defined like this:

class User {
  private String email;
  private String userId;
  private String username;
  // getters and setters...
}

Let say you have a reference to the node users, you can iterate through the nodes as follows:

reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot child : dataSnapshot.getChildren() ){
                // Do magic here
            }
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {}
    });

Note that the DataSnapshot child inside the for loop will have the UIDs as key, not users.

FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference();
myRef.addValueEventListener(new ValueEventListener() {
 @Override
 public void onDataChange(DataSnapshot dataSnapshot) {
    for(DataSnapshot item_snapshot:dataSnapshot.getChildren()) {

      Log.d("item id ",item_snapshot.child("item_id").getValue().toString());
      Log.d("item desc",item_snapshot.child("item_desc").getValue().toString());
     }
  }
}