Menu Items (enable & disable) with parent and child form

Hey, i'm new here and i'm in seperate NEED of help. How do i allow users to only open the same child form only once in a menu item? meaning that if the user already has opened the child form, the user cannot re-open the same child form until the user closes the already opened child form? Eg: once ChildFormA is opened, i have to disable the menu item till ChildFormA is close than enabled the menu item again. Please help me. I'm really stuck. Thank you SA