The accident happened Wednesday afternoon after their family took a break to cool off during a hike. Park officials say the 10-year-old got caught in the river's current at the Vernal Fall Footbridge and was pulled downstream. Efforts to revive him failed.
Searchers Thursday were looking for the 6-year-old, who was either a step-brother or half-brother of the deceased child. The names of the children and their family members were not released, but they were from the Los Angeles area, the San Francisco Chronicle reported.
A national park spokesman said the placid-looking Merced River flows all year long in the park, and eddies and pools mask strong underwater currents.