Watertown Fire Rescue was dispatched to a small grass fire in the Derby Downs Rodeo Complex at 1845 hours Thursday evening.  Upon arrival crews found a small 10ft x 10ft fire burning in the grass and cedar bushes along a fence row.  The fire was extinguished.  The fire was started by young people playing with fireworks.  WPD officers were also on scene.  There was no damage to property.  Crews were on scene less than 20 minutes.