Page numbers are going to differ slightly from edition to edition, so finding the exact starting point and ending point for this question is tough; therefore, I am going to use a quote that is from page 103 of my edition. That should definitely be within the page range provided. The quote I chose is the following:
What is it? he said. What is it? The boy shook his head. Oh Papa, he said. He turned and looked again. What the boy had seen was a charred human infant headless and gutted and blackening on the spit. He bent and picked the boy up and started for the road with him, holding him close. I'm sorry, he whispered. I'm sorry.
This is by far one of the grossest and lowest points for many readers. The two protagonists have discovered an abandoned camp, and there is what remains of a cooking fire. The Man goes to check the perimeter for safety. When he returns, he discovers his son has seen something that no person should ever have to see. They have discovered that whoever was at the camp previously was roasting a headless human baby over the fire. This isn't the only reference to cannibalism in the book, but it is by far the most disturbing reference. I would write the journal entry about this event.
Before writing your journal entry, you need to make some decisions. The first decision is whose "voice" is narrating this journal. Is it your voice? Are you writing about your reactions to this particular sequence, or are you supposed to write a journal entry from the perspective of either the Man or the Boy? The narrating perspective will force you to write the journal differently.
Your question mentions themes, quotes, and story details. The above quote contains all three. It is obviously a quote, and it contains some graphic details. Additionally, it relates to themes about humanity, violence, mortality, good vs. evil, and even survival and perseverance. My recommendation for your journal entry is to pick a theme or two and discuss how that headless baby ties into the theme. Try to keep your entry organized, but don't be afraid to go with a bit of stream of consciousness, too. It is a journal entry, after all.
You might start by discussing how horrific the scene was and how it is a sign of humanity's fallen nature in this bleak world that now exists. Human life simply isn't as sacred as it once was. Move the journal entry from emotional disgust to a more rational and logical thought process. You have no idea why that child is dead. Biologically speaking, the tissues of that child are a viable energy source for a starving person. Historical events, like what happened to the Donner Party, do show that cannibalism can mean survival. Have your journal explore personal thoughts about what may have happened to the child and why the body is still there in the first place. Where did the other people go? You could also discuss your personal thoughts about whether or not you could personally be pushed that far.
Because it is a journal/diary entry, don't be afraid to ask questions in the piece. Answer some of them, and leave other questions unanswered. A goal of a diary is to organize and express inner thoughts, but those aren't always clear. Answers are not always readily available, so make your diary entry reflect that.