I cannot recommend two-up on a scooter, especially with a child because it throws your balance off. A nurse in Paris killed a tourist when she was riding 2 up a few months ago. Maybe an e-bike with a seat over the rear tire? I own scooters but for rare 2 up occasions that’s the setup I use. As an option buy the kid their own scooter & cool looking helmet and you’ll both have more fun!