#!/bin/bash

curpid="$$"
#launch 2 useless child processes
cat /dev/random > /dev/null &
cat /dev/random > /dev/null &
cpid=`pgrep -P $curpid`  && echo "$(basename $0) pid: $curpid; child pids:" $cpid
#kill the child pids
kill $cpid
# check if any child pids still exist
newcpid=`pgrep -P $$`
if [ $? -ne "0" ] || [ "$newcpid" != "" ]; then
  echo "no child pids left..."
else
  echo $newcpid
fi

#!/bin/bash

curpid="$$"
#launch 2 useless child processes
cat /dev/random > /dev/null &
cat /dev/random > /dev/null &
cpid=`pgrep -P $curpid`  && echo "$(basename $0) pid: $curpid; child pids:" $cpid
#kill the child pids
kill $cpid
# check if any child pids still exist
newcpid=`pgrep -P $curpid`
if [ $? -ne "0" ] || [ "$newcpid" != "" ]; then
  echo "no child pids left..."
else
  echo $newcpid
fi

Output:

{0} 02:34:57] $ ./test3.sh 
test3.sh pid: 7015; child pids: 7016 7017
./test3.sh: line 12:  7016 Terminated              cat /dev/random > /dev/null
./test3.sh: line 12:  7017 Terminated              cat /dev/random > /dev/null
no child pids left...

If you launch any child processes as another user (e.g. sudo), then you may not have permission to kill the process even though you are the parent. If you change one of the

cat /dev/random > /dev/null &

lines to

sudo cat /dev/random > /dev/null &

you will not be able to kill that process (assuming you launched the script initially with a regular user account)

Modified script (running one child as root and outputting diff info at end):

#!/bin/bash

curpid="$$"
#launch 2 useless child processes
cat /dev/random > /dev/null &
sudo cat /dev/random > /dev/null &
cpid=`pgrep -P $curpid`  && echo "$(basename $0) pid: $curpid; child pids:" $cpid
#kill the child pids
kill $cpid
sleep 0.5
#check on children
for i in $cpid; do
  echo -n "PID: $i; Orig PPID: $curpid; Cur PPID: "`/usr/bin/ps --ppid $i | grep -Eo '[0-9]{3,}'`
  echo
done

Output with one of the children running as root:

[{0} 02:53:51] $ ./test3.sh 
test3.sh pid: 8144; child pids: 8145 8146
./test3.sh: line 9: kill: (8146) - Operation not permitted
./test3.sh: line 10:  8145 Terminated              cat /dev/random > /dev/null
PID: 8145; Orig PPID: 8144; Cur PPID: 
PID: 8146; Orig PPID: 8144; Cur PPID: 8150

The child PID launched that was launched as root no longer considers the initial script as it's parent, instead it's the subshell created with the sudo call. The output from ps auxf
shows this clearly as well:

root      8146  0.0  0.0 244996  7444 pts/2    S    02:54   0:00 sudo cat /dev/random
root      8150  0.0  0.0 113828   744 pts/2    S    02:54   0:00  \_ cat /dev/random

This issue isn't the crux of your question, but its something to keep in mind.