There were a few things, that Yavanna did not know about Saeril. If not many. And maybe that was a good thing. Yavanna probably would haven't have liked the thought, of someone prying into her most inner thoughts. But what she didn't know, can't hurt. Kili and Yavanna acted like siblings...teasing each other. Both were young and innocent. Kili enjoyed teasing her, if only to make her feel bit better. But of course, they both knew when to be serious and act their age...if not maybe beyond their years.