Просто сделай это
DatabaseReference all = FirebaseDatabase.getInstance().getReference().child("All");
all.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
//Loop through all children
for (DataSnapshot child : dataSnapshot.getChildren()) {
String key = child.getKey();
String value = (String) child.getValue();
}
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
}
});