I agree with cards to the cafeteria. My friends had no money when their 5 yr. old son had cancer and less than usual due to her taking a leave of absence because of his illness. As you can imagine someone was with him all the time. I think a bag for the parents is a great idea, books, gift cards to the cafeteria, itunes, borders or so they can buy there own books, small blanket, snack food, longer lasting fruits like apple and oranges, drinks, breath mints, toothbrush, gum)
What really helped my freinds and costs little money is dinners for their freezer over time. It really helped my friends to have meals in their freezer. Between work, hospital, and taking care of other child it's hard to fit in shopping and cooking.
My daughter is 11 and I'll tell you that overnight she went from having her walls filled with Jonas Bros./Miley posters to Twilight posters. So double check what she likes. Mine still loves the Tween gossip magazines that are in the larger grocery stores.. On chemo she maybe tired some days with enough energy to read smaller artlcles.
If she has a IPOD and somewhere to charge and load songs an itunes card would be good so she can get audiobooks, movies, music and tv.