It is a shame she herself left the scene of the accident and did not call the police immediately. They would have attended, taken down all the details your daughter could provide, and then searched for the offending vehicle and driver. Any evidence at the scene such as car body parts would also have assisted the police in their enquiries.
Instead your daughter also left the scene I of an accident and so now has been left to prove the accident even happened. A real pity as some what's it has got away with an offence of leaving the scene of an accident and maybe others like drink driving along with traffic infringements. Is the owner of the car disputing who was driving, along with apparently not being involved at all?
This makes the job for the police to bring a prosecution very difficult indeed and they need irrefutable proof, like witnesses who you say do not exist, or perhaps CCTV if available (?)
Not a good situation but thank God your daughter and baby is ok.