Orn,
I'll just check, you are aware of CALL ui.Interface.getChildInstances(childname)
https://4js.com/online_documentation/fjs-fgl-manual-html/User/ClassInterface.html#METHODS (second from bottom). The demo MDI example FGLDIR/demo/Containers/Main1.4gl uses it. So instead of "opening just another Application A" you can prevent another instance of application A opening.
The other thing you might want to look at, I experimented with it at my previous employment before I joined FourJs AP, so I don't have the source, it might not work with MDI windows and it might not be appropriate for Windows Vista and 7. Look up AppActivate and VBScript. The concept being create a little VB script that is something like ...
Set WshShell = CreateObject("WScript.Shell")
WshShell.AppActivate "Title of your Child Window"
... send it to the client via FGL_PUTFILE() and then execute it via shellexec.
Also a similar technique may require using the Windows Windows Id. The frontcall getwindowid can be used to get that and then passed into a similar script
Hope that helps,
Reuben