Police say a 28-year-old woman is now in police custody in relation to the attack at the Tencreek Holiday Park in Looe, Cornwall, at about 5am on Saturday.
The boy, who has not been named, was found dead by police at the scene following the attack by the dog, described as a “bulldog-type breed”.
Devon and Cornwall Police launched a search for the woman dog owner and her pet following the incident, eventually arresting her in Saltash in south-east Cornwall just after 8am on Saturday.
The woman is being held in custody on suspicion of having a dog dangerously out of control and manslaughter.
The dog was also found and has been transferred to kennels.