According to the novelization, Dawn is "staying with her friend Janice" during the events portrayed, something Buffy is grateful for given the threat level.
“Let’s go.” She moved ahead, hefting the bag of Slayer goodies she’d
grabbed at her house in her left hand. She hadn’t had time for a
shower, but a few extra weapons were never a bad idea and she took
care of it. Dawn wasn’t home, and for a moment she’d been terrified.
Then she remembered that her little sister was staying with her friend
Janice. It was a risk having Dawn away from home, but on this night
that wasn’t exactly a bad thing either. Her sister needed protecting,
and better that she be with a regular family for the night than be at
home alone when Kakistos was out and roaming the streets.
The Buffy wikia, however, disputes the canonicity of the novel as it states that the events take place in the sixth season while the prequel comic states that they take place in the fifth season and "Since the comic was written by the same writers as the game, Christopher Golden and Tom Sniegoski, this setting should take priority."