There are two ways to manage riders. You can manage them in the account, or in the carpool.
At the account level, the riders listed are "My Riders" i.e. my children and you can add or remove children from your family / account. This makes them available to participate in different carpools. To manage children in your family / account:
At the carpool level you can enable or disable riders for that particular carpool from your family. To do this: