Multithreaded program and fork(): alternative or s

In a multithreaded Linux/C++-program, I want to use fork() with a signal handler for SIGCHLD.

In the child process I use open() to create two new file descriptors, sendfile() and close(), then the child exits.

I planned to use fork() to implement the following requirements:

The threads in the parent process shall be able to

detect the normal termination of the child process, and in that case shall be able to create another fork() doing the open()/sendfile()/close() for a range of files
kill the sendfile()-child process in case of a specific event and detect the intentional termination to clean up

For requirement 1 I could just wait for the result of sendfile(). Requirement 2 is why I think I need to use fork() in the first place.

After reading the following posts

Threads and fork(). How can I deal with that?
fork in multi-threaded program

I think that my solution might not be a good one.

My questions are:

Is there any other solution to implement requirement 2 ?
Or how can I make sure that the library calls open(), close() and sendfile() will be okay?

Update:

The program will run on a Busybox Linux / ARM
I've assumed that I should use sendfile() for having the most efficient file transfer due to several posts I've read regarding this topic. A safe way to implement my requirement could be using fork() and exec*() with cp, with the disadvantage that the file transfer might be less efficient

Update 2:

it's sufficient to fork() once in case of a specific event (instead of once per file) since I switched to exec*() with rsync in the child process. However the program needs invoke that rsync always in case of a specific event.

You can use threads, but forcefully terminating threads typically leads to memory leaks and other problems.

My linux experience is somewhat limited, but I would probably try to fork the program early, before it gets multithreaded. Now that you have two instances, the single threaded instance can be safely used to manage the starting and stopping of additional instances.