Awwlook, as I said, nothing will be done unless OP has filmed it.
We're people of good standing, both have had/do jobs where we needed Enhanced CRBs (now DBS) - yet apparently, any one individual's word is not enough to secure a conviction even if, like us, you are staid middle aged, well educated people with no criminal record, and the people mistreating the animal have, like my neighbours, a record stretching back years.
Nonody's word is good enough any more. There has to be hard evidence.
When I saw the incident, my phone was downstairs and the whole thing was over before I would even have got to the door - although I was sort of frozen to the spot, in horror.
I have also alerted the SS and NSPCC (who already have input with this family) what I saw as their younger child was present, and about a foot or two away as the dog was being laid into by her parents. They didn't even make a house call. These things are put down to 'malicious neighbours' - maybe because it's cheaper and easier than actually investigating.