OR state police say the man was standing on the shore holding his children when they were swept up by a wave.
An officer who arrived at the scene found the father struggling to get out of the ocean and the girl further out in the ocean, Oregon State Police said.
Despite the search, captured on video by KPTV, had to be suspended at sunset and the little boy was not located.
"Stay away from these unsafe surf conditions", he said. The girl was pronounced dead at the hospital, according to the report. "We're obviously hoping for the best, but at this point it's very challenging".
Officials have not yet released the names of the three or provided an update on the status of the father.
A Manzanita Police Department officer dove into the surf to rescue the girl, KATU reported.
The coast was under a National Weather Service high surf warning, the highest alert for waves, and was also experiencing unusually high tides known as king tides.
Jeremy Stiles, 47, of Portland and his daughter were taken by ambulance to a hospital in the nearby city of Seaside, while the Coast Guard deployed helicopters to help search for the boy. "People can be swept off rocks and jetties and drown while observing the high surf".
On Saturday, high tide hit the area around 11:40 a.m. and reached 8.3 feet.