It happened at Dinner Creek on Sunday night near the 6400 block of Barker Cypress Road.
According to a tweet posted by the Cy-Fair Fire Department, a 4-year-old boy was treated by paramedics and was rushed to the hospital by Life Flight. He's in serious condition, according to the tweet.
Cy-Fair FD said three other people were evaluated at the scene but refused treatment.
It's unclear what caused the accident. The incident remains under investigation.