My daughter has a Neon that experienced a failed timing belt. As expected it bent valves and caused serious damage to the head. My question is - how much damage can be done to the lower part of the engine (pistons, crank etc.)? Assuming that there is at least cosmetic damage to one or more pistons, is there the possibility that it is only cosmetic?

The bottom line is risk vs. cost. The car has many new parts and should provide reliable transportation for several more miles if the engine was running.

Damage to the top of pistons should be a pretty good gauge to any other possible damage, such as to connecting rods. If piston damage appears cosmetic when the head is removed, everything else is likely okay. I doubt you damaged the crankshaft or journals.