Refresh drawing on another child form

In MDI parent form (MDIForm) I declare two child forms:

// child form1
ClassForm1 frmForm1 = new ClassForm1();
frmForm1.MdiParent = this;
frmForm1.Show();

// child form2
ClassForm2 frmForm2 = new ClassForm2();
frmForm2.MdiParent = this;
frmForm2.Show();

frmForm2  contains "graph drawing".
How do I refresh "graph drawing" on  frmForm2 , if I, for example, click on a button on frmForm1.

thx