All you have to do is ask one of the conductors at Main St if you can ride in the Lilly Belle. If they have enough conductors to spare one to chaperone, they will let you ride.
We had dined at Club 33 and had heard that if you dined there, you could ride in the Lilly Belle. When I asked the conductor and had my Club 33 tickets ready to show, she said that anyone can ride.
We rode with Bob and he was great about telling us lots about
Disneyland and the train.