It's not a job I could do, trt. Social workers don't even have the right of entry if a child protection order is in place, in fact they have very few powers.
At lot can happen in the time it takes for the police to arrive, ladybirder. A child can be taken out through the back door or driven away, evidence hidden, Calpol administered to make the child drowsy.
If the police are called and they force entry and they find nothing amiss any trust between the social services and the family will be broken. Forcing entry could aggravate the situation and be traumatic for the children. The police will only force entry if the social worker believes the child is in immediate danger so it's a very difficult situation.
Why not have an obligatory health check every, say, three months until they reach school age ? It's seen as a great benefit to ensure the child remains well, rather than a big brother check on the parents, and ensures no long term abuse. It need only take a short while each. How exorbitant would be the cost ?
How far would you want such an exam to go, OG? Examine genitalia for signs of sexual abuse?
Physical exams can show physical abuse but not emotional abuse which can be just as damaging.
I really don't there are the resources to do this, and in any case abusive parents would find ways to avoid them.
I wish I had the answers, ladybirder. Social workers are viewed either as heavy handed, power mad Gestapo who do more harm than good by taking children away from their families; or left wing, clueless do-gooders who do nothing.