There is one Parent class & n number of child classes. But at a time only one child class only should access my parent class.. How to achieve this?

OK, so it would be retarded for a tunnel to be derived from a car. Instead, each car, when it gets to the tunnel, needs to check first if another car is in it. The instance of tunnel will return true until the car in there leaves, and until it does, your code won't let the new car enter the tunnel. which means you'd have a queue of some sort, of cars waiting to enter the tunnel.

Impossible. Every child class IS an instance of the parent class. Accessing has nothing to do with it, it IS the parent class. You'd have to create some sort of blocking mechanism, by hand, or rethink your design as a whole. Your best bet would be to edit your post and tell us why you want to do this.