Store multiple nodes to Firebase Realtime Database

I'm new to using Firebase and Android Studio and I need multiple nodes to save to a realtime database in Firebase from my android app, these include users, budgets and spending as it's an expense tracking app. I've got the user information to save under its own node but I can't figure out how to create nodes for budget and spending. The code below is what I'm having difficulty with :

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_categories);
    setupUIViews();
    firebaseAuth = getInstance();
    mDatabase = FirebaseDatabase.getInstance().getReference();

    btn_subCat.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(validate()){
                sendUserBudgets();
                Toast.makeText(Categories.this, "Completed!", Toast.LENGTH_LONG).show();
                finish();
                startActivity(new Intent(Categories.this, Menu.class ));
            }else{
                Toast.makeText(Categories.this, "Submission failed", Toast.LENGTH_LONG).show();
            }
        }
    });
}
private void setupUIViews() {
    travel_input = (EditText)findViewById(R.id.travel_input);
    entertainment_input = (EditText)findViewById(R.id.entertainment_input);
    fitness_input = (EditText)findViewById(R.id.fitness_input);
    beauty_input = (EditText)findViewById(R.id.beauty_input);
    clothes_input = (EditText)findViewById(R.id.clothes_input);
    holiday_input = (EditText)findViewById(R.id.holiday_input);
    food_input = (EditText)findViewById(R.id.food_input);
    mobile_input = (EditText)findViewById(R.id.mobile_input);
    btn_subCat = (Button)findViewById(R.id.btn_subCat);

}

private Boolean validate() {
    boolean result = false;
    travel_budget = Double.parseDouble(travel_input.getText().toString().trim());
    entertainment_budget = Double.parseDouble(entertainment_input.getText().toString().trim());
    fitness_budget = Double.parseDouble(fitness_input.getText().toString().trim());
    beauty_budget = Double.parseDouble(beauty_input.getText().toString().trim());
    clothes_budget = Double.parseDouble(clothes_input.getText().toString().trim());
    holiday_budget = Double.parseDouble(holiday_input.getText().toString().trim());
    food_budget = Double.parseDouble(food_input.getText().toString().trim());
    mobile_budget = Double.parseDouble(mobile_input.getText().toString().trim());

    if(travel_budget.equals(null) || entertainment_budget.equals(null) || fitness_budget.equals(null) || beauty_budget.equals(null) || clothes_budget.equals(null) || holiday_budget.equals(null) || food_budget.equals(null) ||
            mobile_budget.equals(null)){
        Toast.makeText(Categories.this, "Please enter all fields", Toast.LENGTH_LONG).show();
    } else {
        result = true;
    }
    return result;
}

private void sendUserBudgets() {
    String currentUserID = firebaseAuth.getUid();
    CategoriesDB catDb = new CategoriesDB(travel_budget, entertainment_budget, fitness_budget, beauty_budget, clothes_budget, holiday_budget, food_budget, mobile_budget);
    mDatabase.child("User").child(currentUserID).child("Budgets").setValue(catDb);
}