R.K.
You may be able to take the blame off you and still get your way. Call DMV or the police department and ask if there is an age limit on riding in a sidecar. If it's against the law well it's against the law she can't go. I'm not sure that your dad is undermining you but I do think he is being sneaky about the whole thing. I would insist no matter what that she have a helmet and protective clothing. I know most motorcycle drivers are safe and careful but not all drivers of cars are. I know if it were my child I would allow it.