/bin/cat :Bad file descriptor

Question

I am pretty new to the pipe(), fork() functions and /bin. I would like to pass a value through a pipe from my first child to my second child that I will later use to change the value. However, I thought that if I make line and just line++ in the second child, it would be okay. However, line would then become a shared value. So I used /bin/cat to properly call out the stdout but I can't seem to get it to work.

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<string.h>
#include<sys/wait.h>
#include<stdio.h>
#include<unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#define READ_END 0
#define WRITE_END 1
 
int fd1[2];
char line[100];
int main() 
{ 
    int line = 1;
    char *args[] = {"/bin/cat",NULL};
    //child 1 process 
    if(fork() == 0)
    {   
        close(fd1[READ_END]);
        dup2(fd1[WRITE_END], WRITE_END);
        printf("Num 1: %d\n", line);
        close(fd1[WRITE_END]);
        exit(0); 
    }
    // child 2 process 
    if (fork() == 0)
    {
        close(fd1[WRITE_END]); 
        line++;
        dup2(fd1[READ_END], READ_END);
        printf("Num 2: %d\n", line);
        close(fd1[READ_END]);
        execv("/bin/cat",args);
        exit(0);

    } 
    else if(fork() != 0);
    // Write input string and close writing end of first pipe.
    close(fd1[READ_END]);
    close(fd1[WRITE_END]);
    exit(0); 
    return 0;
}

My output is:

Num 1: 1
Num 2: 2
/bin/cat: -: Bad file descriptor
/bin/cat: closing standard input: Bad file descriptor