Do you mean when you maximise the Child window it make the Main form maximise? That would be odd.
Or
When you click maximise on the minimised child window it fills the parent client area, if so thats what its supposed to do!
When the child form is minimised you should see a Maximise and a float icon (left hand icon replacing the minimise) if you click the float icon it should go back to its previous size and position.
Well it does in my main Mdi form Application.
(Written originally on 98 but now running under Vista, I haven't tried it on 7 yet, that might be the problem?)
If you are not seeing the float icon perhaps you are setting up the border icons incorrectly, if you do this in code can you post it?
Steve: N.M.N.F.
If something is popular, it must be wrong: Mark Twain