The average you could expect to recieve from a brand new battery, with a 50 pound kid on pavement, should be a around45min to an hour. Here comes the it depends part. If the battery is older, then not as long. If the kid is lighter/heaver, then longer/shorter respectively. If it's ridden in grass/uphill/rough surface then shorter run times could be expected.
Speed, Distance, Torque... Pick any 2