Chiming in a bit belatedly, but if you're after candids, you might want something a bit wider than 50mm. At the very least you'll want something fast.
This would eliminate the DA*55 (as awesome as the lens is, you'd probably find yourself using the 18-135 more often than not except in dark situations). You'd be amazed at how much you might miss that extra 5mm range with uncooperative subjects on the fly.
I'd suggest maybe grab a (cheap) manual or AF 50mm and grab the DA 35mm f/2.4 to go along with it. The 35mm is still fast(ish) and probably would be a lot better at grabbing shots where your kids are running on and off frame then what you'd find with a 50mm.
In my own experience my 50mm shots (all manual mind you, I have no AF 50) tend to be when my kids are limited in movement due to sitting down or sleeping. Once my oldest is running around, its time for the 18-55 or 28-90 (or heck, even the Sigma 70-300) to come out. The ability to zoom in and out to grab them/frame them/etc becomes very important indeed, and may even be just as or even more valuable than the AF.
Since you have the 18-135, this means wider may be better as far as primes go simply as it gives you more wiggle room when your subject is running back and forth and generally not cooperating for a candid. You'd find yourself probably using the 50 more as a 'creative' lens then a quick snapshot lens.
So my suggestion: Whichever 50mm is cheapest from your list (probably the DA 50 unless you find a bargain on the used market), and add the 35mm to your arsenal to go along with it since the 35 would probably suit your needs more as far as candid shots go.
EDIT: To give you an idea of how much more useful the 35 would be for candid, this was shot at 35mm on the kit lens. The hand is mine. and I was holding the camera at eye level with my other hand. A 50 would be just too narrow for this. Once I got inside the laundromat to get him running around the place, I swapped over to the DA 18-55 and then the FA28-90 due to the fact he was running around and I started flying all over the map with my focal length shooting him.
EDIT II: The more I think about it, pairing the new DA50 and the DA35 together just sounds tasty indeed. That said... I'd be far more likely to have the 18-55 on my camera in a situation where I would see myself needing both.