It's a very big deal from a Catholic perspective, but shouldn't (and hasn't been, in my experience) a big deal for gift giving. Truly, a nice card with good thoughts and prayers would be lovely. Any gift you'd like to give in addition to that would be more than appreciated, I'm sure. Some ideas in addition to money, an "older" children's bible, another cross necklace (i have at least 5, and wear them all at different times), cross earings if her ears a pierces, or a book about a saint she might find inspiring (Joan of Arc is always a favorite with young girls). If you go to a Christian bookstore they usually have a First Communion section and certainly have a kids section where you might find something to fit the bill.