without spending too much time writing a long reply, here are my quick comments
- You are mixing too many systems together. The electrical (battery) system affects your top speed, but the drivetrain (motor power, gearbox) also does. Try to think of each system separately and understand each of them before understanding how they each influence each other
- Kid scooter is not a good comparison with a gokart because they are 2 different vehicles. weight is different, traction is different amongst other things
- Ultimately, you didnt mention what kind of top speed you are looking for (as well as other key parameters like weight, acceleration, range, overall cost), so it's hard to really help even if we wanted to.