Five-year-old /*Nicholas Kirby*/ died in May when an oak tree fell and hit the pick-up he was in. His parents were also both injured in the accident.
The $5 million claim says the trees were rotting and the county should have known they were dangerous.
A tree expert ABC7 spoke with at the time found significant decay in almost every tree along the road. He was unable to examine the tree that hit the Kirby's car, but two arborists who did could not determine what caused it to fall.
Sonoma County officials say they may never be able to find out.