The last entry into the Diary of a Madman is the shortest of all entries saying "Maybe there are some children around who still haven't eaten human flesh. Save the children.... This is a very pessimistic ending, the narrator is scared that the world is being corrupted by this group of cannibals and he worries about the next generation....for he fears that it is doomed. There are only two possible outcomes for these children. Either they become cannibals themselves or be eaten by them. The children themselves have no way out.
He is writing this diary for those who are still not cannibals, and those are the only ones who would be able to save the children. What he is calling for is for them to unite, and take a stand against these flesh eaters, figuratively speaking. He is calling for the children to be saved from the oppressive enemy that is killing off their existence in one of the worst possible ways imaginable.