I have 1 MDI form on which there are 2 picture box , when I am opening child form it goes behind the picture box.
Here is the code for opening child form

private void schoolToolStripMenuItem_Click(object sender, EventArgs e)
            classSetup cs = new classSetup();
            cs.MdiParent = this;

Set your main form property IsMdiContainer to true

private void toolStripMenuItem_Click(object sender, EventArgs e)

            Form f1 = new Form();
            f1.MdiParent = this;

is this what you want?