I surmise that the reason why the toddler was found ten miles from the mother is that a friend of the mother, another sex worker maybe, was babysitting the toddler. Maybe the mother texted something that indicated how long she would be, and therefore location, and the killer needed to mop up. I would suggest the babysitter is buried close to the the toddler and the mother was killed sometime earlier.
I also believe there are two killers working together. Killers do not take risks. To park a car beside the road and drag or carry a body to the beach while your car is unattended is a risk. A patrol car might stop and question why the car is abandoned. More likely one person drove the car while the other disposed of the bodies.