Sorry what, "adjust to the move" ? For 8 weeks? Does she fear 'adjusting to the move' when you go away for a fortnight to Spain to? Suggest that local children will tease them for their names? Does she fear you are taking her grandchildren away from her when you go away?
! It's not 'moving', it's "going on a long holiday when their Dad is working abroad". It's 8 weeks or less, that's long holiday. Great that you can do it. Fabulous if he'll get some time off (or tag time off to the start/end of working period) so you can travel around a bit. It's a holiday.
For that length of time, if it doesn't work for you/the DCs, you can just stay in the UK when DH is away/only go for part of it. Bit shitty to be apart for that long, but not outside 'the norm' for many families.
Is your mother prone to drama llama tendancies?
I'd give her something proper to worry about and suggest that you discussed it and agreed the US move might not be the best,it's a lot of travelling back and forth, and there's a role that's full time in the Dubai/Moscow/Sydney office and you feel it would be great to go for all of you, huge ex-pat community so we'd not have to worry about 'fitting in', and DH would be back in the UK office a couple of months a year so able to see the DCs then, after all, a couple of months is a huge amount of time... 