Cod why didn't you phone the Police in the first place ?
'I am so and so JP I am witnessing the verbal and physical abuse of a child right now ' .
they would have blue ligts on and come to you direct . likely to be there when adult was there , get details , may takle action but anyway the details would be passed on to SS via them and Police Child Protection Unit. .
I really dont see what SS will do now . They may know of the family or assume who the famioly is but now yoou have no real evidence, except your word , what happened.
Should have dialled 999 .
But hindsight is a great gift , hope you do get somewher with SS , have found them very slow when use to dela with them on a professional basis .