You'll lose scope to that variable/method. You can only access/invoke variables/methods that are defined in the parent. However if the Child class overrides one of these methods (not variables!) that method will be called instead of the method of the parent.
No. Variables don't contain anything except a reference to an object. And the object always contains the same data no matter what type of variables contain references to it.
However the compiler doesn't let you access anything except variables and methods which are declared for the type of variable. So in your example, the Child object continues to contain everything which a Child object normally contains, but the Parent variable which refers to it can only access variables and methods from Parent.