Touch Your Heart, definitely. I dropped it eventually, but I'm super picky about romance. I can't remember if there's a love triangle, if there is, it didn't leave a big enough impression. You may drop it for the (lack of) plot.
You could also try The K2, which I'm partway through. It has that dynamic. The female lead, (that is the younger female lead) is certainly childish and sweet, and she would be loved... if she'd grown up in different circumstances. The male lead is calm and competent. Primarily, I wouldn't describe it as a romance, but as a political action thriller.
Thirty But Seventeen. The male lead is very distant from others, and the female lead is essentially a seventeen year-old, who wakes up one day to discover she's been in a coma for 13 years and is now 30. Everyone loves her. There's love triangles, but they don't feel particularly... badly written, and the main two are always, without a doubt, the main two.
Kahogo no Kahoko(Overprotected Kahoko) Jdrama. Without a doubt, childish female lead. That's the point. I wouldn't call the ML "Calm", but he's certainly more sensible. Probably one of the better Japanese romcoms of recent years.
Also, and this may be pushing it, but Ms. Hammurabi. It's semi-episodic with a slowburn story, revolving around legal cases. But I absolutely loved the ML's calm, cynical commentary. And the female lead, though not childish, is outgoing and loved by the people that matter. EDIT: There's a love triangle, I think, but it was so without a doubt not going to happen that I forgot.
These suggestions cover a diverse range of genres, hopefully there's something to your taste.
