Ben,
I just read your request after doing some research where we are trying to offer a solution for this sector. You can try a 3rd party service that captures legal binding e-signatures through esigtek platform. With this you can customize your permission slip and send it through the system via email so then the parent can verify and validate this information and only clicking to sign and voila. The cost is inexpensive specially if you compare it to the amount of time and money you will be saving from this.
let me know if you are interested