I suggest you should take "CAR CAB service" as they have their terminal in main Quetta city at Saryab Road. Sadabahar bus Service is good but not good for families. u can easily get car cab from old subzi mandi near HASCOL Petrol pump on road both side u can see there shop branding in karachi.
or u can booked car from khi - quetta - ziarat - khi or You may hire rent a car service from Quetta to reach Ziarat.
There are many options available at Ziarat for staying, You may hire the complete guest house in around 3500 per day but most preferable is PTDC motel. Do bargain with them for room charges and negotiate at around 2000 to 2500.
Availability of water would be a big issue in winter season. The water get freezed in taps and you must check the availability of geyser and hot water at your hotel both in Quetta and Ziarat.
Take along with the warmest clothes you have. Do take some additional blankets with you for kids.
Main markets remains open till 7 to 8pm.
Quaid residency is open for public from 10am to 5pm.
There is no water at Hanna lake and would be useless to visit there. Rest is your choice.