I will be hiring a car in Sofia to drive to Borovets - will I be able to drive to the top of the mountain and do shuttle runs to save the family having to pay for lift passes?
You can drive by car to Hotel Jastrebez and your family can use these run down to the centre of Borovez, but jastrebez hotel is only on 1650m over sea level Borovez centre 1350m. So not a good idea.