Dollar Bill Spoof

Hey I just got a job at our library reading to kids. THey have this program where kids get prizes for reading. I was asked to make a spoof of the dollar bill as a certificate for this prizes. How much difference do I need to make this legal? I have some very obvious differences, but is there anything that should NOT be on it? I have Serial Numbers off, have "THis bill is not legal tender" 3 places, changed some pictures. Anybody know the rules on this?

That's really all you need man. Print it out on different color paper to make it interesting though. All it has to say is This Bill is Not Legal Tender or Void or something like that... I'd put something on there like "Library Currency" or something to remind kids it's exclusive to that library and they can't go buy Pokemon with it.