Harry Potter and the Goblet of Fire
Goblet of Fire is definitely still kid-friendly (at least, sexually speaking; otherwise, it's kind of violent). We don't see much more than Fleur Delacour and Roger Davies making out in the bushes during the Yule Ball. All the same, there are hints of love affairs to come: Viktor Krum takes a real liking to Hermione, Ron is in denial about his giant crush on his best friend (Hermione, again), and Harry has fallen for Cho Chang from afar. And while she barely appears in Goblet of Fire, we still see hints that Ginny Weasley continues to think Harry's the best thing since sliced bread.