This can get sticky, since there are hundreds of international abductions every year. If you truly believe it is for a cruise, you might consider it. You can ask for the written itinerary and documentation showing that they are really going on the cruise. Generally the abduction of children comes as no surprise to the abandoned parent. You could agree to have the passports kept in a safety deposit box that requires two signatures to retrieve. If your ex has citizenship in another country, you might want to do research to find out about whether that country is a member of the Hague Convention and whether you could retrieve the children if they were kidnapped. You should seek legal advice if kidnapping is a real concern.