VS 2012 Refer to controls on a child form from a non child form

I have a parent form named ParentMenuF. I have several different child forms. One of the child forms has data that I need to be able to manipulate from another form. I have done this before when not using parent and child forms. Now that I am using parent and child forms, I'm not sure how to get this done.

1) How do you manipulate data on one child form from another child form? (ChildForm1.MyTextbox.Text = ChildForm2.SomeField.Text)

2) How do you manipulate data on a child form from another form that is not a child or parent form? In other words, it's a regular form by itself not attached to any parent. (ChildForm1.MyTextbox.Text = RegularForm2.SomeField.Text)

Thanks