how can I manage the forms from the Parent Form

I have application with 3 forms , Parent and 2 child forms with names fristForm and secondForm.

On parent form at public Form()  I open the firstForm. as follow :

firstForm _firstForm = new firstForm(); // Code on Parent 

public Form1() {
InitializeComponent();
firstForm _firstForm .MdiParent = this; 
_firstForm .Show(); 
}

from firstForm I call function from Parent Form as follow : _

 public void _closeDb(firstForm _formReceive)
        {
           _firstForm = _formReceive;
           _firstForm .Close();  
// Open the SecondForm 
scondForm _secondForm = new secondForm();
_secondForm.MdiParent = this;
 _secondForm.shwo();

        }

But the scondForm not Open , Can any one help be to open the form

Best Regards

Swalf