I too looked at the cheaper options for a speedlight trying to get over the sticker shock. I was talked out of it by the good folks here at Nikonians and bought an SB700 and have not looked back.
IF you are either knowledgeable about manually setting flashes or are prepared to invest the time to become so, then you may save you pennies and go for an off brand. IF most of your flash shots are going to be in similar environments then I suppose you can play around with the settings and learn what works and what does not. You'd want to learn at least the best setting for your typical indoor shot in your house in the daytime (lots of light from the windows) and after dark when the only ambient light is you house lighting. And you'd want to learn how to do fill flash with the off brand. That too is useful.
IF not, then the ttl (through the lens) auto feature of an Nikon speedlight is a great thing. I use that almost exclusively. That is worth the extra $ because you do not end up with completely washed out over exposed photos.
As for CLS (Creative Lighting System) that too might be a great thing, but until you buy a second flash you're probably not going to be using the CLS. You might want to read up more about it. It looks like a cool thing. Your 3100, like my 5100 does not include a built in commander mode to control one Nikon flash off the camera. I know the SB700 attached to my camera has this command mode built in, so it effectively adds this CLS capacity to my camera, I'd need to buy a second (and third) Nikon flash to get the maximum benefit of three flashes controlled by the one on your camera using the TTL exposure control feature. Lots to learn there.
IF you do not see yourself getting into CLS and just want better snapshots of your child using something better than the built in flash look at the SB400. It is a step up, does not cost a fortune and does have the TTL feature. I do not thing it works in the Nikon's CLS lighting system.