Before I got my own passport and was still travelling on my mother's passport I made my own! I got the inspiration from a school project and I was about seven at the time. I got it stamped on my numerous trips to LAX each year when my father was working there and pretty much whenever I went abroad I requested the stamp. Most immigration folks thought it was cute and stamped happily, the only time it was refused was flying into Spain once (can't remember if it was BCN, AGP or MAD ) and I got very upset!

Can't imagine the INS folks at Dulles being too happy about stamping a made up passport for a 5yr old these days...