My first thought was that the textbook was a bigger security risk than the craft project. The craft project sounds like it boils down to drawing/writing on a few glued together papers and the textbook is this bigger, set object, where one could easily slip money between the pages. Granted, craft-girl could glue money into her project, but it sounds like it was laid out, straight forward and possibly easier to monitor with security cameras, which I assume tellers have. I don't know your bank layout, but many around here have the drive thru tellers separated from the rest, so it seems like they wouldn't get as much direct human supervision, which means one just has to 'beat the security cameras'.
I don't know how security is run at banks; 1) if there's a person watching the cameras, 2) if someone reviews the footage after the fact or 3) if it's just stored on a 'as needed' basis, but 1) security could have told your supervisor that they couldn't fully see what you were doing with the textbook so that's why you were told or 2) security told your supervisor that craft girl shouldn't have been doing things at her station, hence why you were told... since you said a few weeks passed between the incidents.