Lisa,
First off, your friend is blessed to have such a caring 'Girlfriend' back home thinking of and taking care of her!
Secondly, I have had the opportunity to travel to Mexico and Spain in the past: (oddly enough, I never learned Spanish), anyway...simple American pleasures are best. The things that I took with me - that were life savers was facial tissue (the soft Kleenex kind) and toilet tissue! (preferably Northern) A bit bulky to ship, but light and very much appreciated by your friend and her family.
Third, on a sweet note - send peanut butter! (plastic jars) Ritz crackers, gummy bears for the girls, anything princess...stickers, color books, reading books. Do they have access to a DVD player? Maybe on their lap top...send a few kiddo movies and maybe a chick flick for your girlfriend and then something that she and her husband would enjoy together when the girls are in bed.
I think that one of the other Ladies which responded gave a great idea of 'Don't try to send it all the first time' -space it out - and from me - don't really try to set yourself a schedule to send things, that way when (not if), but when you are unable to keep that schedule of sending items, you do not start beating yourself up for messing up.
Just some thoughts from a Mom who has been married to her college sweetheart for 30+ years. Someone who has had the opportunity to travel and learn from her own experiences, as well as from others. Blessings to you Lisa, for caring for your friend.