when we moved o'seas one january everyone just got us the normal presents ans we posted them to our new address in the new year as all of our removals had already been shipped. it was quite cool as the dcs got christmas twice, effectively 
would sis like some of the union jack cushions that seem to be ubiquitous? a little something to remind her of 'home'? pic/ painting of a special place from where she was brought up? (we move all over the world and i have some pictures of my home town on the walls) and i do have a huge union jack rug... which i wouldn't have in the uk lol.
the children might like some us dollars in a card to send when they get there? that way they can buy themselves something nice in case their shipping hasn't arrived
my kids don't seem to be pining for anything english - the niece who is 6 might like an 'american girl' doll though. dd2 is 7 and is desperate for a 'maple lea girl' doll which seems to be the canadian equivalent. i like them because they aren't pink barbie-esque etc. the maple lea girls come from different canadian provinces, not sure if the us ones come from diff states, but it might be cool to get her one that 'matches' where they are heading to if they do...
my friend emigrated to oz this year - last christmas i bought her and i matching photo albums, and a book 'a year of mornings, 3191 miles apart' and suggested that in 2010 we took pictures all year and gave each other the album for the following christmas... so she's in oz and i'm in canada, and our families are going through huge changes - it seemed a nice way to keep in touch with what has been happening - the stuff we can't see, at least once a year?