I had an interview with one of the international law firms. It lasted an hour and created more questions than answers - wills are extremely complex when you have assets in different countries and they all have different laws (Japan complicates things greatly for me as their inheritance laws are nothing like Western laws).
You have to consider the order in which the parents die, where you die, when you whether the children are adults at the time of death (if not, how to allow the guardians to access funds) and all the permutations on how assets acquired in future will change matters... Additionally you might want to create an optimal structure to maximize allowances and minimize taxes.
In the end I gave up and filed it in the "too difficult" category. I'll revisit it when I'm feeling brave enough and we have some more clarity on where we'll be living when we're in the "Death Zone" haha.