Hmm. Hadn't thought of that approach.
I usually just ask any lefty who is sputtering with indignity over Reagan's cruelty to children if he or she really thinks Presidents make decisions personally about what goes where in school lunch programs. Then I ask what he thinks happened when the issue came to Reagan's attention. He won't know, so I tell him -- Reagan reversed the policy. So if Reagan didn't set the policy and reversed it when he found out, then what exactly, I ask, did Reagan do wrong? Generally at this point the lefty changes the subject, but if not, I inquire innocently if he knows that salsa was categorized as a vegetable in school lunch programs during the Clinton maladministration...
And of the rare individual who seems intelligent and open-minded, I ask this: how did you come to believe that this incident reflects badly on President Reagan when the facts obviously don't support it? This segues nicely into a discussion of media propaganda techniques...
Reagan of course had nothing to do with this trivial incident, which was blown up into some kind of crazed assault on children in the press reporting to the sheeple. The decision to classify ketchup as a school lunch vegetable was made by some minor passive-aggressive bureaucrat, most of whom are sc*mb*g beltway democrats anyway.