OCALA, Fla.  - An Ocala father has been killed and two children seriously injured in a tree-trimming accident.   The Marion County Sheriff's Office says Theodore Visneski was using a hydraulic lift boom that trims trees. He was riding in a bucket when it rolled over a septic tank, causing it to topple over. The three fell about 40 feet to the ground.  The oldest child had severe head injuries. The younger child had a broken leg.