It happened around 9 p.m. in the 13000 block of Veterans Memorial near Bammel North Houston Road.
The deputy constable was southbound on the road and was responding to a call about an officer who needed assistance, authorities said. The cruiser had lights and sirens activated when a car pulled out in front of it.
The cruiser slammed into the car and the impact involved three other vehicles. Three people inside the car were taken to hospitals, including the 12-year-old.
The deputy constable was treated at the scene for bumps and bruises.
There was no word on the conditions of the three people hospitalized.