Mainland China is the busiest crossing and the least forgiving on paperwork. A child travelling on a Home Return Permit is inspected on both sides, and cross boundary pupils are checked daily, so those families keep a standing letter with the escort. Where the child holds a foreign passport, the visa rules apply in full and the letter is best prepared in English and Chinese.
Macao applies lighter formalities, yet ferry operators and crossing staff still ask an adult who is not the parent for written authority. Airlines are the other gatekeeper: carriers at Hong Kong International Airport apply their own accompanied minor policies, and a letter that fails to name the accompanying adult can stop a family at check in.
Commonwealth destinations are the strictest. South Africa requires evidence of parental consent for minors and has turned travellers away over an unsigned or undated letter. Canada publishes a recommended consent form, while Australia and the United Kingdom leave it to officer discretion, which means the letter is requested whenever surnames differ.
The Schengen Area treats the letter as evidence for the visa application as much as for the border. Consulates in Hong Kong ask for the consent of both parents where a minor applies for a short stay visa, and several want the signature certified.