The video, taken by a nearby teen, shows the boy screaming as he plunges 27 feet into the water below. The 22-year-old mother is heard cheering from the water as he is tossed off the railroad bridge.
The child landed "straight on his upper back" in the water, according to the Kaylub Fawley, who posted the video on Facebook on Aug. 25.
"This is what happens when some parents just don't care," Fawley wrote.
When the mother talked to officials later, she played off the incident as fun, according to authorities.
The 35-year-old man who tossed the boy was also cited for reckless endangerment and for second-degree criminal trespass for being on the trestle, which is private property, according to the Grays Harbor Sheriff's Office.
The boy did not suffer any injuries but was shaken up, according to authorities.
It's not clear whether the two adults have entered pleas.