I can't really think of anything else possible?
A signed contract??
I do understand where he's coming from, but I kinda think what you have said should be enough.
I can't think of anything else that might help, but I really hope you sort it out & the kids can go.
My SIL is Maori & I'd love to take our family over there one day when the kids are a bit older.
Does he think about how amazing it would be for the kids to see another country?