Accessing control in a child form from another child form in an mdi application

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C#

I have an mdi form with three child say form1 form2 an form3 the childs are accessd through a menustrip from the parent form.i want that whenever u open form1 by the click of its toolstripmenuItem and click a button in it, i want that another button in another form say form2 should b enabled witout showing the form2 unless u click on the form2menustripitem before u will see the change, that is ,i want the change to occure directly on the form and not a new instance of the form2.please i need help on this codes will b very helpful Thank in advance