Not sure if you've already had your exam, but I wouldn't use cy pres, which applies to modify trusts (not wills). I would argue that the gift to the ACS was based on either mistake or delusion. If you could prove that the decedent reasoned from an incorrect fact (that S had run away or was still alive, etc), you might be able to wipe out the gift to the ACS which was dependent on that incorrect fact. because the ACS gift was a residuary that failed, you'd claim that the intestacy statutes should be followed, and petition the court to find that S was dead and S's children are the heirs at law.
Just a guess, based on Florida's statutes, not New York's. This probably wouldn't work, but it's the argument I'd make.