Your kids will think it's an ordinary bag of cheese puffs...
but when they open the bag, they'll discover:
All you need to pull this off is:
individual-serving-size bags of cheese puffs
baby carrots (about 6 for each bag you're making)
slip of paper with the note: "April Fool!"
Open the snack bags carefully. Dump the contents of the cheese-puff bags into a sandwich bag. No sense wasting a perfectly delicious snack! Use a damp paper towel to pick up any crumbs left in the bag. Add about 6 baby carrots--and don't forget the note! Then run the glue stick along the very top inner edge of the bag. Press to seal, and pack in the lunchbox!
I wish I'd be able to be there when my kids find these.
UPDATE: My daughter just found hers when she opened her lunch. I got a text message: "Mom this is just gross. I mean carrots? In cheesecrunchy bag?" (For the record, she likes carrots. It's not like I put in her most un-favorite food ever.) She's threatening revenge.
And Little Brother never noticed: he gave his snack to another kid who had forgotten to bring a snack. And she never said a word. (Kind of disappointing, really!)