What happens if you make an agreement with the “other” parent that they make go abroad for a temporary period with your child(ren) and they then break that agreement and remain there?
This was considered by the Supreme Court In the matter of C (Children)  UKSC 8Supreme Court Rules.
This case addressed the concept of ‘Repudiatory Retention’ In child abduction cases
Following the recent Supreme Court’s decision, parents should be very wary about permitting a child to move overseas, even if the move is temporary (this especially applies if the destination country is not a signatory to The Hague Convention). This is because it is possible for a child to become a habitual resident of another country, even if the left-behind parent vehemently objects.
Unfortunately, it is extremely difficult to determine at what point in time a parent makes a decision to break/breach any existing agreement regarding children being temporarily moved abroad.
If you are a parent and believe the travelling or potentially travelling parent may decide to remain permanently in the destination country, you must seek legal advice. This is because there is a real risk you could lose the opportunity to bring an application under the Convention.