Your car will consume about 8000 worth of Petrol from Kamalia to Murree and back. So you are left with 12k budget for food, accommodation and other activities.
You can search on the spot for different budget hotels but be careful if the hotel is suitable for family or not. You will find one at around 2500 per day for 2 nights thats 5000 for hotel.
Now you are left with 8000 for food and other activities. You will have to ration your budget in food because its something which you can control yourself.
If you decide to visit Pindi point chair lift or even Patriata chair lift, then its additional Rs.500 ticket per head.
Regarding traveling, you should start early. Since you will soon touch M3 from Faisalabad. You won't have traffic issues except for Rajana or Samundari if any. I think if you start at 7 then you can make it to Murree by 2-3 PM. Once you be there, search for a good hotel and settle for the day. In evening you can roam on Mall road and even go to Pindi point on foot and enjoy chair lift.
Next day you can visit Patriata and enjoy chair lift + cable car. But keep in mind that there is huge crowd there on weekends and long queue for number.
On 3rd day, you can leave Murree and reach Islamabad. You can visit Rawal Lake (Lake view park) and boat ride. There is a huge walk-in bird aviary at Lake View Park and its ticket is only 60 Rs per head. Trust me on this one, your kids will love it.
If you still have time and budget left, you can visit Daman-e-Koh which is also a great place to visit and children will like it.
Regarding Mehran capacity to reach Murree and other technical issues, anyone else can guide you better because I'm not best on this one. Normally Mehran will easily make to these points.
Have fun bro.
