School Portraits D5100

[Cait]

I wasn't really sure where to put this, but I thought I might get the best advice here! I recently bought my first SLR, and although I'm still learning how to use it, I have been asked by a relative to go do portraits of some kids at a school. I'm hardly a professional, but no real professional will agree to volunteer their time for free, and these kids (as well as the school) are really stint on cash. I thought it would be great for their parents to have at least one nice photo of their kid, and it would help with the school's growing system.

So here's my question (finally): what advice would you give to a new photographer, to give these kids a great photo to take home? I want it to look as pro as possible, but obviously my resources and equipment is limited (I'm a student). I'm not looking to do this beyond a hobby, so I'm not keen to invest in studio equipment. All I have is a Nikon D5100, a 35mm f1.8, a tripod and an SB-400 flash. Are there any specific settings I could set my camera to? Any specific flash setting I could use? It will have to be indoors, because the weather is so unpredictable here. Are any particular skin tones affected differently by different settings (are there any settings that are more flattering for one skin tone and not another)?

I just want to give these kids and their families great shots, instead of lackadaisical snaps that any old camera could do.