It may be the most simple of answers but it is also fair to say that the Police should be involved where allegations of child abuse are concerned as soon as possible. However, as easy as it may be to say much may also depend on the context.
Where the situation is urgent or in an emergency then the only guidance that can really be given is to involve the Police immediately.
Otherwise it may be that how something is dealt with depends on whether you are the victim of abuse (whether a child or in relation to historical allegations) or whether you are receiving the information from a victim.
For a victim of abuse it can be incredibly hard to find the confidence to come forward. Once that strength has been found it must be remembered that the Police can only carry out a meaningful investigation if they have enough information available to act. Taking the first step is a considerable challenge but if the Police can be involved at an early stage than it may help the investigation. It may be easier to locate the people involved and to gather evidence. One of the great challenges in cases of historical allegations is gathering together evidence and this holds true both for civil and criminal cases.
For someone receiving information it may be necessary to look at the situation differently. For example, a child may tell someone they trust about allegations of child abuse. Much as it may be difficult to betray a confidence it must also be clear that there are times when that must be overcome in order to protect a victim of abuse. Professionals such as social workers and doctors may have protocols about how and who to report to but otherwise it would be recommended that as soon as there is enough information available that would allow the Police to investigate or assess things then that is the point that they should be involved.