I think the themes discussed in the WWI material I did for this site are understandable to ninth graders. Certainly Wilson’s double standard, recognized by international lawyers everywhere, with regard to the British hunger blockade and the German submarine warfare. I just came across this article, by a guy who favors a vigorous prosecution of the War on Terror (and so is not a dogmatic noninterventionist), that you may find useful: http://20committee.com/2015/04/24/woodrow-wilsons-great-folly/
World War II is trickier, but I would note how mainstream opposition to intervention was. Pat Buchanan discusses this in A Republic, Not an Empire. Even the moderate of all moderates, Gerald Ford, was an enthusiastic recruiter for a campus America First Committee. Herbert Hoover favored staying out. JFK donated to the AFC. The New York Times’ military expert, Hanson Baldwin, said in his 1949 book Great Mistakes of the War, that it would certainly have been better to let the two totalitarian powers fight it out.
I would stress that we are dealing with 50-60 million dead people, and that for that reason, it’s obviously not out of bounds to think about alternate scenarios — unless we’re going to claim 50-60 million dead was the best possible outcome.