if your budget allows, you can buy both the 35mm f/1.8 and 50mm f/1.8 so that you can try out the working distance. hope that helps.
just to share with you my experience of taking photos of my nephews, with my D90 and both AF-S DX 35mm f/1.8 and AF-D 50mm f/1.4.
Auto-focus speed with the AF-D 50mm f/1.4 was faster than the AF-S DX 35mm f/1.8, so the AF-D 50mm f/1.4 was better at capturing my nephews running around. However, the field of view when used indoors is too restrictive.
sorry to hijack this thread... but its on the same topic..
I think the Nikkor 50mm f/1.8 would be a good buy, good lens for the price. If you have some spare cash but don't wanna burn a hole in your pocket with those high-end primes, then you can also take a look at the Tamron 17-50mm f/2.8 or 28-75mm f/2.8. With some range for zoom, it'll be a little more flexible during shoots.