what you want is a parent constraint. you can then key an attribute in the constraint on and off (it actually has a floating number so that when you have the object parented to two objects, you can actually have it slid between the two instead of snapping back and forth).

For example, Create a sphere and a plane. select the sphere, then shift select the plane. In your animation set, select constrain > parent. move the sphere around, the plane follows with it. now select the plane. Under SHAPES, you should see " pPlane1_parentConstraint1 ". If you select that, a few attributes become visible, the bottom one should be "P Sphere1 WO", it is this attribute that you can animate to turn the parenting on and off. the name of this attribute will typically be the <name of the parent> WO

hope this helps