Callgrind reacts on requests to the parent in the child process. Requests done 
with callgrind_control are execued by the parent, the child or both.

Here is a testprogramm:

#include <cstdlib>
#include <cstdio>

extern "C" {
#include <unistd.h>
#include <sys/types.h>
}

int sum = 0;

int main() {

        if (fork() == -1) {
                perror("Could not create child");
                exit(-1);
        }
        while (true) {
                sleep(1);

                for (int i = 0; i < 100000000; ++i) sum += i;
        }
}


Compile it and run it with valgrind --tool=callgrind -v ./a.out

Then iterate

callgrind_control -i off <PID of parent>
callgrind_control -i on  <PID of parent>

and you might see something like that in the logfile:

--12409-- Command: instrumentation switched OFF
--12409-- Command: instrumentation switched ON
--12409-- Command: instrumentation switched OFF
--12409-- Command: instrumentation switched ON
--12409-- Command: instrumentation switched OFF
--12408-- Command: instrumentation switched OFF
--12409-- Command: instrumentation switched ON

Here 12408 is the pid of the parent and 12409 the pid of the child.