Try 貝澳沙灘. It is managed by government. It is safe with 漁農署 staff station in the camp site. You have to register for camp space reservation. It is quite professional management. Many restautants nearby but just too far away from the city. It takes more take one and half hour from the city but it would be good suggestion for summer time.
You can also consider 鶴藪營地. You can park your car next to the camp site. At the morning, you can spend time in 鶴藪水塘 and 流水響水塘. They have lovely view. There are few 家樂徑 around. Your family will have fun time there.
Here is some sample pictures for 貝澳營地.
http://www.southernscout.org/scouting/outdoor/campsite/puio/graph
It is well organized camp site. It is much better than other. It just 5 mins walk from the bus stop with prefect view.