Best DSLR + lens combo for low light, indoor kid shots

Due to the release to the D5200 I think one of the best value deals for your situation would be to snap up a remaining D5100 Body ($500) 35mm 1.8g DX Lens ($200) for indoor close quarters, a Nikon 50mm 1.8g -effectively 75mm - ($250) for a little more reach and nicer bokeh in portraits, and a Sunpak PZ42X flash ($150) to fill in when needed.  Also a compatible flash diffuser ($20).

This combination will take very nice pictures without breaking the bank and give you some flexibility while retaining very crisp images in low light.  While these may not be the most advanced or newest options i think they offer a very good value/quality ratio.