Hi Lisa! Thank you for asking such a great question!
Rider Switch is really a great service and the Walt Disney World Resort had families in mind when they created this program! To answer your question: No, the adult waiting with the child who does not wish to, or can’t ride does not need a fastpass.
Rider Switch is available at select attractions at all four theme parks. It works like this; before entering the normal queue you will speak with a cast member and notify them that you wish to use Rider Switch. The cast member will issue you a Rider Switch Pass and ask you wait while the rest of your party experiences the attraction. After they ride the attraction, your party will locate you, and you will then “switch.” You will now become the rider and enter the attraction by giving a cast member your Rider Switch Pass. You won’t have to wait in the regular queue a second time!
Also if you are looking for some helpful hints for traveling with children, be sure to check out the Planning for Pre-School Kids and Little Ones page on the Walt Disney World Resort website. It is packed full of information and helpful hints on making your little ones dreams come true!
I hope that this has helped to better explain this pixie dusted service! Please come back at any time and ask me your question! Have a magical family vacation!