Just click on the Repeat field, and then click the box where it will say "none" if there is no repeat set. A dropdown box should appear, and the option at the bottom should be what you are looking for.
Anders, I'm still unable to get that option. I still have the standard options for the radio controls such as Repeat from Due Date and Repeat from completion date. I don't see any other option related to the parent. Is there something I'm missing?