Child abduction occurs when a parent takes a child outside the country, often to avoid custody dispute, without the consent of the other parent. There are some dispositions in international law that can be used. If you child has been taken from another country to Thailand or from Thailand to another country. It is more frequent that you think.
Thailand has ratified The Hague Convention of 25 October 1980 on the Civil Aspects of International Child Abduction. This multilateral treaty is an international convention, wishes to protect children from abduction and harmful effects of retention across international boundaries. You can see on this following link that Thailand ratified this treaty on 14 August 1982. However, it is not because a country ratified a treaty that its application will be strict and efficient. Each country can apply these disposition at their own rythm. It could be difficult to enforce even if the country signed the treaty.
The USA GOVERNMENT website specifies that American citizens travelling abroad are subjects to Thai Courts and foreign orders about child custody:
“ENFORCEMENT OF FOREIGN ORDERS: Foreign orders (including U.S. custody orders) are not enforced/enforceable in Thailand. American citizens who travel to Thailand place themselves under the jurisdiction of Thai courts. If a taking parent chooses to remain in Thailand with a child or leave a child behind in Thailand, the U.S. Embassy cannot force either the taking parent or the Thai Government to return the child to the United States. American citizens planning a trip to Thailand with dual national children should bear this in mind.”

The first thing you should do is to contact the authorities (police, government offices, embassies, etc.) in case of abduction. There are some agencies protecting children in most countries
Once you have contacted the authorities, you could need a lawyer in order to start some legal proceedings. Sometimes, the authorities do these cases without lawyers. You just want to be sure that things are being done properly and quickly. You can also use Thai Courts and try to get full and sole custody if you don’t. That would give you all the rights on the child but it is hard to get here. Even if your child is abroad, a clear decision by a Thai court about custody here could help you.
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- For USA, consult: https://travel.state.gov/content/travel/en/International-Parental-Child-Abduction.html
- For AUSTRALIA, consult: https://www.ag.gov.au/FamiliesAndMarriage/Families/InternationalFamilyLaw/Pages/Internationalparentalchildabduction.aspx
- For CANADA, consult: https://travel.gc.ca/assistance/emergency-info/child-abduction-welfare
- For UK, consult: https://www.gov.uk/government/publications/international-parental-child-abduction
- For FRANCE, consult: http://www.enlevement-parental.justice.gouv.fr/
- For GERMANY, consult: http://www.bundesjustizamt.de/
- Countries part of the Hague convention: https://www.hcch.net/en/instruments/conventions/status-table/?cid=24