The only way is to not have the child frame parented to the main frame. You'll need to adjust the frame level to what it would be if it were parented to make sure it draws on top. To make it show/hide with the main frame, you'll need to hook the OnShow and OnHide scripts on the main frame to call
frame:Show() and
frame:Hide() on the "child".
For frames "MainFrame" and "ChildFrame" do:
Code:
ChildFrame:SetFrameLevel(MainFrame:GetFrameLevel()+1);
MainFrame:HookScript("OnShow",function() ChildFrame:Show(); end);
MainFrame:HookScript("OnHide",function() ChildFrame:Hide(); end);
This should mimic the Show/Hide behavior while isolating the alpha. Note, ChildFrame
cannot be parented to MainFrame, otherwise the alpha of MainFrame will be applied.