If you can send your students home with books, it's a great way to keep them interested and motivated going forward. (And yes, kids do still read books!)
Donated books are the best. No Starch Press has been very generous with donations for PyCon over the years - it's worth contacting them to see if they'll help out. Otherwise they have a great group purchase program (email email@example.com for more details).
Some titles to consider from No Starch:
Python for Kids: A Playful Introduction to Programming
by Jason R. Briggs
Learn to Program with Minecraft
by Craig Richardson
by Mahesh Venkitachalam
Another great title for younger students is Hello World! Computer Programming for Kids and Other Beginners by Warren and Carter Sande. The publisher, Manning, also has a discount for group orders.
Finally, if you can't get book donations, be sure to recommend a few follow up projects for your students to tackle when they get home.