Investigators said the bomb was left at the checkpoint Thursday night by two men on a motorcycle.
The child and an adult were treated at a hospital, the news website Colombia Reports said Friday.
The men on the motorcycle fired warning shots and ordered civilians to leave the area before the bomb was detonated, police said.
The city of Neiva, in southcentral Colombia, has been the site of recent attacks by FARC rebels, a Marxist revolutionary organization, the website said.
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