Best to tell him asap- if he seriously objects he could in theory file an urgent court hearing between now & Xmas flagging their passports and preventing the trip from happening. Could be a delicate operation gaining his consent- maybe a 'hey would it be ok with you if I took the kids on an overseas trip' to test the waters before telling him that it's actually happening. Also be prepared that he may want his turn to take the kids on an overseas trip without you.