Hi Venkat,
Since the Idocs are being processed at the child system. It seems the issue is with your reverse RFC set up, as the acknowledgement is not sent to the central system. Test your reverse RFC and see if there are any connection problems. Check out WE02/03 and find out if there are any idocs pending at the child system.
Even if this doesn't work, then try disconnecting the child system completely through RSDELCUA and try reconnecting it again.
Let us know the results.