Earlier, officials said the child was 12.
The child's other three siblings, a 7-year-old boy, an 11-year-old boy, and a 13-year-old girl, were overcome by the water and did not survive.
A deputy saw the girl and tried to help, but couldn't save her.
"He was one of the first ones out here as well and was able to dive in the water and help pull out the young female. He's a father as well. We're all devastated. Wish we could do more," Harris County Sheriff Ed Gonzalez said.
Veronika Alvarez is the one who saved the 10-year-old. She said she was sitting on a bench at the beach, when her dog started barking.
She didn't pause to think and immediately ran to the rocks.
She says the kids were on one side, and their parents were on the other side. The current was strong, and it may have swept them out.
"I saw the parents were trying to grab them but then I saw the mom and dad go under. I just did a jump in the water... go straight for the boy drowning. I didn't know it was super deep where he was, where everyone was drowning, so I kept him up but the current kept pulling us both down, so I knew to just keep him up. I didn't care if I drowned or not, I just knew to keep him up," Alvarez said.
Alvarez said she saw the mom was drowning as well and struggled with who to help first.
"I had the mom drowning as well. I was like, 'Who to grab?' I knew the kid was the first responsibility," Alvarez told ABC13.
She said she didn't realize at the time that multiple children were in distress. Otherwise, she would have tried to save them, too.
"I thought the other hands I was feeling were the people around me. I didn't know there were other kids going down. I would have tried my best. I found out at the hospital there were three other ones and I just started crying," Alvarez said.
Alvarez had bruises from where the boy was holding on, trying to stay afloat. "He was just holding on for dear life. It was just something tough, the rocks were coming at my legs, which is why I have socks," she said.
Alvarez, who is a veterinary assistant, did have to go the hospital. She was treated and released around 1 a.m. Friday. The boy she saved was treated by EMS on the scene and released.
She said she doesn't consider herself a hero.
"I know a life is a life. It depended on someone jumping in rather than everyone just standing. The officer was there. I was yelling, shouting. No one was responding, everyone was quick to grab their phones and I was like that's not important. What's important are the people that were drowning," Alvarez said.
Investigators say the family had just moved to the Houston area.
ORIGINAL STORY: Woman saved boy whose 3 siblings drowned while swimming at Sylvan Beach in La Porte
Woman saved boy whose 3 siblings drowned