strerror(errno) used after waitpid. what should be expected result from strerror?

Question

I tried with strerror(errno) get the waitpid() result string.
I checked the status string in both conditions:

1)  Child is running by adding `sleep(30)` in child process
2)  Without `sleep(30)` in child

I got the below displays from strerror():

While Child is Running:

WAITPID/WAITID returns: 0
WAITPID/WAITID childStatus: 0
waitpid: Success 

Child Terminated:

WAITPID/WAITID returns: -1
WAITPID/WAITID childStatus: 0
waitpid: No child processes

What should be the expected result string from strerror(errno) after waitpid(). What do you mean be "No child Process" does it a valid string if child is exited/terminated normally? What should be expected return value from waitpid() if child is running / child terminated?

Here are my cases of strerror() returns:

"Success"               : in case of child is running with sleep in child
"No child processes"    : in case of child is terminated without sleep in child

Code used:

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

int main(void)
{
    pid_t Checksum_pid = fork();
    printf("\nChecksum_pid : %d", Checksum_pid);

    if (Checksum_pid < 0)
        printf("Fork Failed\n");
    else if (Checksum_pid == 0)
    {
    printf("\nInside Child Process before execl\n");        
    //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 = 0xFF00;  
    siginfo_t si;      
    printf("\nInside Parent Process");
    
    printf("\nParent Sleep for 5 second");
    sleep(5);
    
    printf("\nchildStatus : %d", childStatus);
    printf("\nstrerror : %s", strerror(errno));
    pid_t returnValue = waitpid(Checksum_pid, &childStatus, WNOHANG);
    //pid_t returnValue = waitid(P_PID, Checksum_pid, &si, WNOHANG | WEXITED);  
    printf("\nreturnValue : %d", returnValue);
        printf("\nchildStatus : %d", childStatus);
    printf("\nstrerror : %s", strerror(errno));
    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");
        }

    if (WIFEXITED(childStatus)) {
            printf("exited, status=%d\n", WEXITSTATUS(childStatus));
    } else if (WIFSIGNALED(childStatus)) {
            printf("killed by signal %d\n", WTERMSIG(childStatus));
    } else if (WIFSTOPPED(childStatus)) {
            printf("stopped by signal %d\n", WSTOPSIG(childStatus));
    } else if (WIFCONTINUED(childStatus)) {
            printf("continued\n");
    }
    else{
        perror("waitpid error");
    }

    printf("\nParent: I am about to finish\n");
    }

    return 0;
}

O/P:

Checksum_pid : 17459
Inside Parent Process

Checksum_pid : 0
Inside Child Process before execl
a.out  ___waitpid_test1  waitpid_test1~  waitpid_test1.c  waitpid_test1.c~  waitpid_test2.c  waitpid_test2.c~
Parent Sleep for 5 second
childStatus : 65280
strerror : Success
returnValue : 17459
childStatus : 0
strerror : Success
Child Exit Code: 0
exited, status=0

Parent: I am about to finish

O/P without sleep:

Checksum_pid : 17486
Inside Parent Process
Parent Sleep for 5 second
childStatus : 65280
strerror : Success
returnValue : 0
childStatus : 65280
strerror : Success
Child process still running
exited, status=255

Parent: I am about to finish

Checksum_pid : 0
Inside Child Process before execl
ubuntu:~/Test1$ a.out  ___waitpid_test1  waitpid_test1~  waitpid_test1.c  waitpid_test1.c~  waitpid_test2.c  waitpid_test2.c~