No, I do appreciate that, and I'm sorry if that appeared insensitive. I would be in bits if they were my children, too. I was just hoping that it might be of some small comfort. And I do genuinely feel that, enormous loss as it would be to both, the relationship could and would remain incredibly strong, despite grief on both sides.
I agree instructing a solicitor with expertise on this area might be your best bet. Is there no way he can talk to her (via mediation, if necessary) so he can highlight the loss to the kids of both parents in their daily lives? And is there an argument on her side that she has a great deal more emotional/practical support herself there, and/or better career prospects?
The New Zealand and Australian school year has 4 terms, each 10 weeks long, with 4 holidays. If the worst does happen, and as you say she will be on a good salary, and you can manage 1 flight annually for them yourself, your DH might be able to argue that he gets every Christmas (because that's the sole long holiday - 6 weeks) and all the rest, or at least 3 of the holidays, because she would have care the rest of the time. But that might not be feasible in terms of your own ability to look after them, I do appreciate that - nobody has 10/12 weeks of annual leave to throw about. However, I also flew to Australia to visit my father after a divorce when I was two, so unless things have changed, airlines will assign flight crew to care for unaccompanied minors and they are treated like royalty - no queues anywhere, first on and off planes, pick of the onboard food for all cabin classes and usually an air stewardess who loves kids and wanted the job. I remember being tucked in at night across the row of empty seats (this was pre-internet making booking super efficient, so doubt it still applies, sadly) they had put us into!
What I am saying is that I agree it would be heartbreaking, and I hope it doesn't happen (and it can sometimes be denied, from the comments below, and obviously a good lawyer will help with that) but if it does, then he will need you to argue for the best possible deal, and how to make it work for the children going forwards.
In case she decides to try to just take them, Reunite are excellent.