Give this a try:
In the code that invokes the second (child form), probably a button click event code method. If that button is called btnLoadChild, then:
private void btnLoadChild_Click(object sender, EventArgs e)
frmChild frm = new frmChild();
Since you've set up the parent as an MDI client, the child form appears on top of the MDI parent form. (I like to set the parent's WindowSize property to Maximum.) You can have a Close button on the child form that calls Close() to unload the form when you're done and control returns to the parent.
See page 393 of my book for additional details if you need them.
Jack Purdum, Ph.D.
Author: Beginning C# 3.0: Introduction to Object Oriented Programming (and 14 other programming texts)