Nick, it really depends on the depth of the scratch.
A superficial scratch can often be 'buffed' out by the polishing materials to the point where a deflection by the laser will be less probable and hence make the dvd readable again. Some superficial scratches are oversampled and the errors by-passed.
If a scratch is really deep, you will have errors with deflection of the laser light and secondly, possible damage to the data layer itself. This damage is irreversible.
The only thing one can do is estimate the depth of the scratch and determine if that data layer has been compromised. If so, toss it. If not, give the polishing materials a go. I recommend using polishing materials made specifically for the polycarbonate cd medium. Anything else and you chance a little experimental failure in chemistry.
I suggest you make backups before the remaining disks get destroyed and give the copies to the kids. Keep the originals for future backup copies.
