Police can remove a child under 'Police Protection' if there is harm or a risk of harm. The case would generally then be passed to SS, who can apply to the Court for an order, or they can get the parents to agree to SS accommodating the child while the situation is addressed.
There are banks of emergency carers who are used to taking children on with no notice, for short periods of time. Carers are extremely stretched these days but even so, if a child is at risk, they will be removed and placed elsewhere.
I think you did the right thing Harman - let the authorities know and they are best placed to investigate the real situation.
Oh, and SS will contact the Police in these sort of cases to see if the Police have had any dealings with the people before, so in a way, it doesn't matter which you call.
I hope the lad is ok - sounds like the family do need support...