How about starting the Chronicles of Narnia collection for them? A neighbor gave DS11 the 1st two books The Magicians Nephew and The Lion, the Witch, and the Wardrobe for his 1st Communion gift. Between them and us he now has the whole series. He was able to read it on his own in 2nd grade, some may not be able to, and as he read, we talked a lot about the religious aspects of the books.
A Bible is a good choice (there are Catholic ones and Protestant ones though). That was always my parents gift to the grandkids. There is a wonderful picture book called The Weight of the Mass by Nobisso that my daughter received. An age appropriate saint book is also a good choice. (Do you see a book theme here?)
If there is a Catholic store near you, they would have lots of things specifically for !st Communion and general gift ideas too.