Got a big GS and ride with my grand-daughter on the back often. She's 11 now, but started at 6 (took a while to reach the pegs, shes a shorty).
With Kids first rule. Make a big deal with everything. Treat them like you are teaching them how to ride. So first time show them all the hot parts, gear required and why (without the drama). Then since they are passenger tell them what they have to do and what you expect from them as a passenger. i.e Mine is so light I like to feel her, so I ask her to squeeze my waist once in a while, I also reach back often for her knees/leg. They love to put their arms out and fly, so tell them it's ok when going slow like right close to the ice cream parlor but on the faster bits ask her to keep a hand on the grab bar by her side or your jacket. This is in case you do have to do a sudden maneuver one day to avoid something, they wont lose their balance. Also get them into the side to side movement, I like to do the slalom at various speeds (never over speed limit), helps them feel loose, nothing drastic, just relaxing.
All it's about is them feeling confident and protected while going out for that special half mile trip to the corner for some goodies. Since it's such a short ride, you can make it a bit extra memorable. And since they are not your kids, this helps loosen up the parents too, so next time it can be a bit better longer ride.
Kids and bikes are awesome, in my case my grand-daughter started riding her own bike at 5 so she was already used to the get up to go out, so it's consistant and the same rules if she drives her own or rides as a passenger on mine.
michel
