I found a cargo net from a car or truck worked great as a child containment devise. If they have the "bunge" cord top you'll need to run a peice of static line thru so it won't stretch.
"Hypocritical AND dangerous" is riding a bike with your son or daughter in one of those rear seats, with or without a helmet.
Maxum 3200 Miss Adventure
Formerly - Tartan 34C Yawl