Hi,
one simple way is to have two mesh, the end of the tongue and the body of the tongue. attach the body of the tongue to the end of the tongue, animate the end of the tongue, and scale the body on the stretch axisfor the other side to stay put inside the frog's mouth.
I use this technic for simple ropes, on a crane for example, I have a one unit rope that a stretch, the down side is that if you have a texture it will stretch, but in your case, since it will be a fast animation, I think you are likely able to get away with it.
bye,
Jean