I love romance in kids books even better then in adult books. Children's romances are always more subtle (a good thing, take note authors of the world), more happy and just more adorable. Enjoy the following mixture of fanart and illustrations.
Disclaimer: Due to the depressing lack of Ember and Casson fanart, I found images that looked right, although are not originally from said fandom.
Howl and Sophie from the Moving Castle series. Diana Wynne Jones can say so much in so few words. You never get sweeping displays of affection, yet Howl and Sophie are one of the most romantic couples ever. They spend more then half of their time bickering, and the rest of it saving lives, while tossing Morgan back and forth between them.
Lina and Doon from the Ember series. There was never any doubting how perfect they were for each other.. but you know, in the first book Lina is like 12. I was a stressful journey getting to the end, because sometimes the author does not know what is best, but it ended perfectly. Once again, subtlety is everything.
Betsy and Joe, from the Bets-Tacy series. They are my all time favorite book couple. The way their relationship grows through the books... It's terribly romantic, yet so gradual, every time I read the books I find some other clue or phrase, and I'm like, of course! Does this make sense? I doubt I could ever condense my love for them into a few lines.
Ron and Hermione from the Harry Potter series. I knew they were destined from the second book, JK threw all that subtext in and you could either pick it up or ignore it, but they were always destined. And then at the end, the Epic Kiss, and it's not surprising, you want to just scream "FINALLY" at them.
Caddy and Michael from the Casson series. They are just crazy and epic. You hear their story completely from other people's points-of-view, so it can be very confusing and sometimes you literally have to decode what is going on between them. Attention to detail is everything in these books. Like, how long did it take to get that Daphne was made up? Or that Caddy was pregnant?
Shasta and Aravis from the Narnia series. They are really the only definite romance in the Narnia books, even though they're so young at the start. But they're totally adorable, and like everyone else on this list, like to fight a lot. Hm. I just wish we had gotten more from them, one book was not enough.
I feel bad for doubting Amy Pond. I'm not a huge fan of sassy female characters, I prefer those who seem to be reincarnated from Civil War generals (Olivia, Scully). But Amy is pretty all-around adorable, and like literally insane. And it took me all of two minutes to forget the name of Tennant. Matt Smith is a sexy, sexy beast.