Footage widely shared on social media showed the aftermath of the attack in a palm-studded resort in the small town of Cortazar, about 65 km (40 miles) south of the Guanajuato city.
It was not clear who was behind the shooting that killed the seven-year-old, three men and three women, Cortazar's local security department said. One person was seriously injured in the La Palma resort, Reuters reported.
But in recent years rival drug cartels have been waging brutal battles to control territory and trafficking routes through the state.
Video taken soon after the attack showed shocked adults and children walking past piles of dead bodies near a swimming pool.
"After the attack, (the attackers) fled, but not before causing damage to the resort store and taking the security cameras and the monitor," Cortazar's security department said in a statement.
MNA/PR