Of course social services can become involved where there no illegal acts (such as neglect) have been proved. Their investigations will surely be part of any criminal process if a parent is being charged with something like neglect.
So I believe that, when there is a report that is of concern - like children being left alone for an extended period - they can and do investigate, to see what is actually happening and whether there is any cause for concern.
If ther is cause for concern, then they will presumably look at various courses of action and decide what to do, and if there is no cause for concern, the case will be closed.
This is exactly what happened when a friend of mine was reported to social services by a malicious neighbour, who alleged that she left her young son home alone. They investigated, found that it was a lie, and closed the case file.
But they will not be able to know if a reported situation is or is not a problem without some sort of investigation - which may show that something illegal,is happening, or that something is a cause for concern, but not illegal - in which case they can offer advice/support etc, or that there is no problem.
In this case, being investigated might put the wind up this mum - who is not taking proper care of her children when she goes off on her holidays leaving them home alone - and make her change her ways.