It's very possible that they did not see your earlier email, or they saw it and meant to do it but forgot.
I would simply send a followup email, worded along the lines of
I am writing as a followup to an earlier email regarding a receipt for the day camp that my daughter [name] attended on [insert date]. This receipt is a requirement for me to be reimbursed from my dependent care spending account for the camp cost. If you have any questions regarding this, please either respond to the email or you can call me at xxx-yyy-zzzz.
I appreciate your response to this, and again if you have any questions or concerns please do not hesitate to contact me.
Thank you for your time!
This is a basic polite request, points out that it is a second followup, and outlines the necessity of having the receipt (as in, I don't get $$ if you don't produce).
As a secondary item, you could also consider cc'ing the administrative email for the school. I would be fairly certain that they would have to have approved the day camp, and if the teacher cannot get a receipt, they may be able to. The only downside is that if the teacher was not approved for the camp or similar, they may get in trouble for it.