I am no expert, but this is what I would have thought:
Scenario 1:
Recommend that the parents report the matter to the Police. If in a mandatory reporting jurisdiction, the elders should separately report it directly to the Police as well.
Separately, the elders should make sure that anyone who is placed in care of kids has the appropriate clearance required in that jurisdiction, including the appropriate no objection from the Police or appropriate authority, just like they do in schools and kindergartens.
Make it clear to the congregation that only those with the relevant clearance are allowed unaccompanied with children.
Scenario 2:
Recommend that the parents report the matter to the Police. If in a mandatory reporting jurisdiction, the elders should separately report it directly to the Police as well. Point out that elders (a)do not have the training to handle the matter, (b) any investigation by the elders may be very stressful to the child, and (c) any investigation by the elders may corrupt the evidence and undermine the ability of the Police to prosecute the matter at a later stage.
Separately, the elders should make sure that anyone who is placed in care of kids has the appropriate clearance required in that jurisdiction, including the appropriate no objection from the Police or appropriate authority, just like they do in schools and kindergartens.
Make it clear to the congregation that only those with the relevant clearance are allowed unaccompanied with children.
Scenario 3:
Recommend to the 40 y.o. that only the Police can handle the matter. The elders can't. If in a mandatory reporting jurisdiction, it is possible that the elders have an obligation to report it directly to the Police as well.
Separately, the elders should make sure that anyone who is placed in care of kids has the appropriate clearance required in that jurisdiction, including the appropriate no objection from the Police or appropriate authority, just like they do in schools and kindergartens.
Make it clear to the congregation that only those with the relevant clearance are allowed unaccompanied with children.
Scenario 4
Recommend to the 40 y.o. that only the Police can handle the matter. If in a mandatory reporting jurisdiction, it is possible that the elders have an obligation to report it directly to the Police as well.
Separately, the elders should make sure that anyone who is placed in care of kids has the appropriate clearance required in that jurisdiction, including the appropriate no objection from the Police or appropriate authority, just like they do in schools and kindergartens.
Make it clear to the congregation that only those with the relevant clearance are allowed unaccompanied with children.
Scenario 5
If in a mandatory reporting jurisdiction, it is possible that the elders have an obligation to report it directly to the Police.
Separately, the elders should make sure that anyone who is placed in care of kids has the appropriate clearance required in that jurisdiction, including the appropriate no objection from the Police or appropriate authority, just like they do in schools and kindergartens.
Make it clear to the congregation that only those with the relevant clearance are allowed unaccompanied with children.