You are entitled to your opinion, but the original readers were chidren to whom the evacuation was a recent and powerful memory. The first ten minutes or so were added to instruct audiences who have almost no knowledge of the war. As for boys in armor, the fact is that medieval armies contained knights who were fas young as ifteen and sixteen years of age. To be sure, they were physically much more fit than thesesoft young actors, which may be what bothers you.
Well, the suits of armour looked ridiculous along with the unicorn Peter was riding (where did that come from?).