Stonemonkey,. I've only just seen your post so maybe by now you've done something.
Please, for this little boy's sake, report this woman. Too many times children end up dead, with neighbours saying that they knew something was wrong but didn't want to interfere.
Parenting is a very tough job, and we are all driven to the end of our tether at time, but what you are describing is beyond reasonable chastisement plus it is systematic. From the sound of it this woman is doing this all the time - can you imagine what that child is goingthrough?
If social services are alerted then they will be able to monitor the situation, and provide suppot and if necessary intervention. Reporting this incident to social services doesn't mean that the children will automatically be taken into care, but it does mean that the authorities are alerted. This child is presumably not yet at school, so it may be that there are no responsible adults he sees who can take action on his beahlf. You know that it's wrong, and you know what you need to do. Please phone social services or the NSPCC, anonymously if necessary. I know it's a hard thing to do but please do it anyway.