The last chapter of book 17: Tonight on the Titanic. Jack and Annie receive a watch to help a dog who was trapped under a spell. However, when they get back home, they realized that the watch has stopped at 2:20 AM: the time the Titanic finished sinking. It's a genuinely sombering scene, seeing two kids trying to process the deaths of hundreds.
In the movie the two kids visit Pompeii. When the volcano erupts a man tries to help Jack. Keep in mind that a lot of people died in the volcano eruption so this nice, caring man might go the same way. He had a family too...
Jack's reaction to losing Annie in the Pompeii chaos is pretty darn sad.
Jack's classmate Tracy comes to him with the script of the play ready, but he changes his mind about being in the play, upsetting her. Poor girl. She has a crush on Jack too. Luckily, by the end of the movie, Jack has made up with her (and is implied to return her crush) and does star in the play after all.
Jack has a nightmare about Annie being stuck outside during a volcano eruption. If you have any siblings of your own, you may be able to relate to Jack.
In the pirate world, Jack's growing self-confidence issues lead him to refuse to take part in the adventure with Annie, causing her to start sobbing (in a massive case of Cry Cute) and run away.
Soon after, Jack regrets his harsh words to Annie after learning that she's been kidnapped by the pirates, and rushes off to save her with heartbreaking desperation.