How do I create multiple entries from one form submission?

Here is the situation:

I need to create a registration form for a kid's program.  The parents will enter their information at the top, then enter their children's information (Child #1, #2, #3).  When the parents hit the submit button, I'd like three different entries to appear on the spreadsheet (one for each child, with the parent info connected to each child).

I could force the parents to simply register each child separately (with a clean form each time), but that would require them entering their general family information each time (not cool).

I did find a thread (http://www.jotform.com/answers/354790-Adding-multiple-entries-for-one-field) that talked about using a custom THANKYOU url with conditional logic that would pull the parent information onto a new form, allowing the parents to simply enter each child's info one at a time.  Each time I tried to use a custom THANKYOU url to pull the parent info I received error 405. 

Any help would be appreciated.

-Steve

Form URL:

http://form.jotform.ca/form/43307266322247