I'll echo David's thoughts about the wide angle - I really should have stated that, but I (instinctively?) picked the only lens that wasn't a "normal". If it's about kid shots, you need fast AF - which basically rules out the 25mm f/1.4 (first version - the Mk II is really quite nice). But the Panasonic 15mm f/1.7 (which would be my pick) is probably too expensive, right? Maybe you can find one used? I really love that lens, and it's fast in every sense of the word. Of the 25mm lenses, I'll stick to my original recommendation: Olympus 25mm f/1.8 - fast, sharp, very appealing rendering (I used to call it my "mini Summicron" - even though the Panasonic 25mm f/1.4 II(!) may fit that description even better; and yes, I also own the original, it's not just wannabe talk).
But here's a thought. Since you already own the 12-100mm, why not think about the new Panasonic 9mm f/1.7? It's a surprisingly great lens, and not that expensive at all.
A word on the Olympus 17mm f/1.8: There's a lot to like about the lens - if you *don't* think of it as a landscape lens. However, optically, it has its irritating limitation at times. As a walk-around or documentary lens (visual notebook), it's fine, AF is great. But aberrations can be annoying; this is one of the (increasingly rare) lenses where electronic corrections can mask its flaws, but also may exacerbate them occasionally. I had it fail to deliver the quality I'd have needed, that's why I replaced it with the 15mm f/1.7, differences in FoV notwithstandin . But again, for movement, street and documentary, the Olympus 17mm f71.8 is well suited.