“Is my son, or we his parents, at legal risk for an accusation of sexual molestation which supposedly happened 20 years ago when he was twelve years old?”
I guess we are talking about some kind of suppressed memory case. Under some circumstances, parents of a minor can be held financially liable for the crimes of their children. This would generally extend to victim restitution. I can find no case or jurisdiction on point that would allow for recovery against parents 20 years after the crime was committed, unless maybe, they were also somehow otherwise liable on an accessory theory.