I'd contact the NSPCC or SS but don't hold your breath. Six of us in this house have heard and seen deeply disturbing things that when we reported them, nothing really came of it and we feel like we are probably down as 'malicious neighbours' - even though my kids recorded something. (They knew we had a recording but never asked for it).
We've been in the same position for 18 months since new neighbours moved in - have heard really disturbing things, over and over. We finally called the SS - they came out, then visited weekly, then dropped the whole thing. Leaving the children still being shouted, sworn at and possibly worse.
In December my teenager said he heard the neighbours screaming abuse followed by a thumping sound like someone or something being thrown against a wall or door, then the child screaming. I hesitated as it had happened when I was out, but then the next day I heard precisely that same sequence of sounds - abuse, thump, scream. So I emailed NSPCC as I heard they kick arse. They emailed back to say it was concerning and they might be contacting the police as well as SS. I used to work with kids, kids from very deprived backgrounds but I had never seen kids spoken to like these two toddlers are.
People visited, man next door was his charming public persona, do-gooders swallowed whatever he told them and vanished.
Leaving the kids in the situation. We've heard both kids beign called fucking cunts, idiots - being told they will be 'belted' and the woman shouted at the kids (toddlers) that if they didn't stop, "I'll knock you out!" Apparently the SS are OK with this.
I am getting to the point I feel that I'd like to sue the man and woman next door for child abuse, myself - for the abuse it is to my children to sit in their living room, listen to this, and know nothing will happen. As no-one cares about those two little kids maybe they will take my kids more seriously. It is deeply distressing.
Currently, like whenever officials have been involved, next door have gone quiet and it's been several months. The other day they did have a riproaring argument and I heard the man shout: "Shut up, or the fucking neighbours will start up again!" They seem to have no awareness that we are just reacting to what we have heard.
Hopefully you live in an area where the SS will actually do something. Here in North Yorkshire, they really don't seem to give a crap. I hope you get the child some help but I really, truly wouldn't hold my breath.