Question: Is it possible to wait for all processes launched by a child process in Windows? I can't modify the child or grandchild processes.

Specifically, here's what I want to do. My process launches uninstallA.exe. The process uninistallA.exe launches uninstallB.exe and immediately exits, and uninstallB.exe runs for a while. I'd like to wait for uninstallB.exe to exit so that I can know when the uninstall is finished.

Answer: 
Create a Job Object with `CreateJobObject`. Use `CreateProcess` to start `UninstallA.exe` in a suspended state. Assign that new process to your job object with `AssignProcessToJobObject`. Start UninstallA.exe running by calling `ResumeThread` on the handle of the thread you got back from `CreateProcess`.

Then the hard part: wait for the job object to complete its execution. Unfortunately, this is quite a bit more complex than anybody would reasonably hope for. The basic idea is that you create an I/O completion port, then you create the object object, associate it with the I/O completion port, and finally wait on the I/O completion port (getting its status with `GetQueuedCompletionStatus`). Raymond Chen has a demonstration (and explanation of how this came about) on his [blog](https://devblogs.microsoft.com/oldnewthing/20130405-00/?p=4743).
There is not a generic way to wait for all grandchildren but for your specific case you may be able to hack something together. You know you are looking for a specific process instance. I would first wait for uninstallA.exe to exit (using WaitForSingleObject) because at that point you know that uninstallB.exe has been started. Then use EnumProcesses and GetProcessImageFileName from PSAPI to find the running uninstallB.exe instance. If you don't find it you know it has already finished, otherwise you can wait for it.

An additional complication is that if you need to support versions of Windows older than XP you can't use GetProcessImageFileName, and for Windows NT you can't use PSAPI at all. For Windows 2000 you can use GetModuleFileNameEx but it has some caveats that mean it might fail sometimes (check docs). If you have to support NT then look up Toolhelp32.

Yes this is super ugly.