You are close with the location. However to ensure the form is placed where you want using the Location property, you must first set the StartPosition to Manual.
Setting the StartPosition to manual will allow the Location property to actually work correctly.
frmCreateClient childForm = new frmCreateClient();
childForm.MdiParent = this;
childForm.Text = "New Client";
childForm.StartPosition = FormStartPosition.Manual;
childForm.Location = new Point(15,15);