Single processes tree with exec

I have to write a script creating processes tree using fork() function. Then I need to use the exec() function inside the script, to generate only ONE processes tree, with pstree -c command.

Ok, so I created processes tree, that's not a problem, let's say:

int main() {
   int pid1, pid2;
   if(pid1 = fork()) {
      printf("%d", pid1);
   } else if (pid2 = fork()) {
      printf("%d", pid2);
   } else {
      printf("%s", "parent process");
   }
   return 0;
}

But how do I use exec function to invoke pstree -c command? And how can I show only one specific tree?

execl("/bin/pstree -c", "pstree -c", 0, 0);

I tried with /bin/, without it...nothing works, the tree is not displayed when I run the script.

And it would display the whole tree anyways, not just the forks I used.

How can I do that?