How to open a new form from another form

In my opinion the main form should be responsible for opening both child form. Here is some pseudo that explains what I would do:

// MainForm
private ChildForm childForm;
private MoreForm moreForm;

ButtonThatOpenTheFirstChildForm_Click()
{
    childForm = CreateTheChildForm();
    childForm.MoreClick += More_Click;
    childForm.Show();
}

More_Click()
{
    childForm.Close();
    moreForm = new MoreForm();
    moreForm.Show();
}

You will just need to create a simple event MoreClick in the first child. The main benefit of this approach is that you can replicate it as needed and you can very easily model some sort of basic workflow.