Prints before execl is not visible in output

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

int main(void)
{
    pid_t Checksum_pid = fork();

    if (Checksum_pid < 0)
        printf("Fork Failed\n");
    else if (Checksum_pid == 0)
    {
    printf("\nInside Child Process before execl");        
    //execl("/bin/sleep", "/bin/sleep", "2", NULL);
    execl("/bin/ls","ls",(char *)NULL) ;
        //exit(EXIT_FAILURE);
    printf("\nInside Child Process after execl\n");
    exit(EXIT_FAILURE);
    }
    else
    {
        int childStatus;        
    printf("\nInside Parent Process");
    sleep(5);
    pid_t returnValue = waitpid(Checksum_pid, &childStatus, WNOHANG);

        if (returnValue > 0)
        {
            if (WIFEXITED(childStatus))
                printf("\nChild Exit Code: %d\n", WEXITSTATUS(childStatus));
            else
                printf("\nChild Exit Status: 0x%.4X\n", childStatus);
        }
        else if (returnValue == 0)
            printf("\nChild process still running\n");
        else
        {
            if (errno == ECHILD)
                printf("\nError ECHILD!!\n");
            else if (errno == EINTR)
                printf("\nError EINTR!!\n");
            else
                printf("\nError EINVAL!!\n");
        }
    printf("\nParent: I am about to finish\n");
    }

    return 0;
}

Output:

kth4kor-2:~/bosch/Test1$ ./a.out  a.out  ___waitpid_test1  waitpid_test1~  waitpid_test1.c  waitpid_test1.c~  waitpid_test2.c  waitpid_test2.c~ 
Inside Parent Process Child Exit Code: 0
Parent: I am about to finish

Now If I will remove sleep(5) in parent then output:

kth4kor-2:~/bosch/Test1$ ./a.out 
Inside Parent Process

Child process still running

Parent: I am about to finish

kth4kor@g3gdev-kth4kor-2:~/bosch/Test1$ a.out  ___waitpid_test1  waitpid_test1~  waitpid_test1.c  waitpid_test1.c~  waitpid_test2.c  waitpid_test2.c~

Finally I tried with 0 as third argument to waitpid instead of WNOHANG then its gave me below output with any prints of child:

kth4kor-2:~/bosch/Test1$ ./a.out 
a.out  ___waitpid_test1  waitpid_test1~  waitpid_test1.c  waitpid_test1.c~  waitpid_test2.c  waitpid_test2.c~
Inside Parent Process
Child Exit Code: 0
Parent: I am about to finish

Can somebody help me to understand the behavior of process after executing execl and before executing it?

Remember that output through printf (to stdout) is normally line buffered. That means the output-buffer gets flushed on newline. As you don't have a newline after the string you want to print, the output-buffer will not be flushed.

Either add a newline last in the string, or manually flush the buffers with fflush.