I do know that when my parents were travel agents, they used to bring jeans/jean-type clothing and panty hose (although they're a little young for those!) because at that time, at least, lots of other countries didn't get those easily. Don't know if it's true now.
As Germany is one of the richest and best developed countries in the world, panty hose is more than readily available everywhere, same goes for basically every other European country, so, please don't bring panty hose, although it would probably be a funny anecdote for the family

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I used to travel to the States a lot (when it comes to goods available at stores, Germany and Austria are rather similar) and the things I bought for my daughter were:
Abercrombie and Fitch, Aeropostale or American Eagle stuff , as it isn't available everywhere here (Hollister seems to be more widely available), American Girl stuff was always a huge hit, especially matching clothes for the doll and the kid (my daughter has an AG doll, but I guess many things would fit other dolls as well), M&M's from the M&M store in funny colors,
bath and body works lip gloss and other glittery stuff, depending on where I went I bought sweaters with sights, like the Empire State Building, on them. Sizes don't matter all that much when it comes to sweaters and t-shirts.
I think it depends if the family is German or American, if they are American they probably miss some special food, if not, it might be nice to try something different, but we, for example, threw a ton of Hershey's chocolate away last year because it tasted really bad to us (like vomit to be exact), so tastes might be different.
Edited to add:
I just saw that they are 1 and 3 not 13, as, for some reason, I read it, so the
Disney store would be a good idea as there are stores in Europe, but they often carry other things than the US stores. With children so small, regular sizes for their age shouldn't be a problem. As summer has finally arrived, how about bathing suits? I usually buy them at Disney stores as they are so cute.