Hood County Sheriff Roger Deeds said someone called 911 just before 10 a.m. Friday to report finding a 2-year-old girl alone near the boat docks on Sandy Beach Drive.
Deputies learned that both the girl and her 3-year-old brother had wandered away from a nearby home. The children's 18-year-old babysitter apparently did not know they were missing.
The boy has been identified as Aaryn Jay Thacker. He apparently rode his bike down toward the dock on Granbury Lake.
A team searched for the missing boy and found him just before 11 a.m. unresponsive in the lake. He was rushed to the hospital, but did not survive.
The teenage babysitter, Briana Delbridge, was taken into custody for questioning and faces charges of injury to a child.
Investigators don't yet know her relationship to the children and what she was doing while she was supposed to be watching them.
Sheriff Deeds said Delbridge has a long criminal history.