The child’s injuries are unknown but are not believed to be life-threatening.
Gwent Police said officers seized the dog and confirmed no other animals were involved.
Chief Inspector Laura Bartley said: “Officers will be making further enquiries at this time and will remain at the scene as the investigation progresses.
“It is possible that you may see ongoing police activity in Caerphilly as part of this work, but please do not be alarmed.
“If you have concerns or information then please do stop and talk with us.”
This is the third serious dog attack for Penyrheol in a year-and-a-half.
Ten-year-old Jack Lis died on November 8, 2021, after being attacked at a friend’s house on Pentwyn.
Jack was killed by an XL American Bully called Beast, which was shot dead by firearms officers after the attack.
Amy Salter, 29, of Llanfabon Drive, Trethomas, and Brandon Hayden, 19, of Pen y Bryn in Penyrheol, were jailed in June last year in connection with Jack’s death.