What border officers actually look for
The letters that pass inspection are specific. They name the child as the passport names them, identify the accompanying adult, state the destination and the return date, and give a phone number the officer can dial on the spot. Vague letters ('I consent to my child traveling') are what generate secondary questioning.
South Africa and Canada are the destinations travelers get caught out by most often; both have published guidance on unaccompanied or single-parent minors. Check the destination country's entry requirements yourself rather than relying on what the airline's call center says.
