If they have passports you need to act quickly and don't think that it won't happen.
My wife abducted my two children and retained them overseas and it is still a long, expensive and very upsetting process for all involved to have them returned home. Whilst NZ is a signatory to the Hague Convention on Civil Matters of International Child Abduction you don't want to have to go down this road if you can avoid it.
This type of thing happens with alarming regularity but it is relatively simple to prevent.
Hide the passports or prevent them from having passports made available and contact the courts and the Federal Police and have both children put on the AFP Airport Watchlist.
Do something now....!
Good luck