My own feeling is that, if you've got the 18-55mm kit lens, then you've already got the best lens for the camera, especially for the subjects you've listed. I wouldn't suggest anything wider than 18mm (the equivalent of 27mm on full-frame 35mm) for family/baby/wedding photography, and the 55mm end of the range is ideal for portraiture.
If you do eventually feel the need for a longer lens, then Nikon's own 55-200 VR lens is incredible value (especially at Amazon's price), and has the advantage of "matching" the 18-55 lens in the sense of the controls (auto/manual and VR-on/off switches, zoom and focus rings) being similarly located on both lenses. Also, if you are buying filters, both lenses have 52 mm filter threads, so you'll only need to buy one set. I've recently bought this lens, and first impressions are that it's very good.
As far as flash is concerned, I would be tempted to wait until I was sure I needed it: I'm biased, preferring 'available light' photography wherever possible. I have to say this camera is surprisingly good at it, even considering that the zoom lenses are not particularly fast. Have a play with different ISO settings: you might be surprised how far you can push it before noise becomes too much of a problem! What I would say, though, is that if you do go for a hotshoe flash, get one with a tilt-and-rotate head so you can bounce light off the ceiling or the wall behind you.
Quality varies (as does price, from a few pounds upwards). My own choice tends to be Hoya, but generally you tend to get what you pay for. My own feeling is that it's a shame to put a cheap bit of glass in front of a pretty good lens!
I have to be honest, though, and admit that I've not put a UV filter on my new Nikon: I bought the Nikon lens hood for it, and it seems to offer pretty good protection. It's made from the same tough plastic as the camera body, and seems to have just the right degree of flexibility to protect the front of the lens barrel from physical damage.
Obviously, if you want belt-and-braces, go for both hood and filter: You pays your money and takes your choice!