What light wattage to use for a nikon D40?

I wanted to buy lighting set to take pics of my kids but what wattage would I buy? Or what would be a good wattage to buy i don't know I'm just a beginner so not sure on these things.

Shawna, The main thing you need to be aware of is that older flash units both camera mounted type and studio type are a different voltage than the flash equipment of today. Today lower voltage units are used. So if you were to buy an older unit, it most likely has too much voltage for a digital camera. Your only choice in this case would be to purchase a Wein Safe-synch @ $50. This unit cut the voltage down so the flash can be used on a digital camera.
That said If I were you I would look into a Alein Bee studio flash available from Paul Buff. This unit sells for about $225 direct from the manufacturer. The do not sell to camera shops or anyone else. They are also very nice to deal with. I have added their web site below. You will also need a light stand and an photographic umbrella I recommend a white or gold. For portraits.
Also a stand alone reflector for the other side of your subject serves as a fill light. As I said, The people at Paul Buff are very nice to deal with and they can help you quite a lot!

Good Luck

You can test light output in the beginning. But may want to get a flash meter if you do enough of this work.

http://www.paulcbuff.com/b400.php

AlienBees B400 Yellow
AlienBees B400 Pink
AlienBees B400 Green
AlienBees B400 White
AlienBees B400 Colors

AlienBees™ B400 Flash Unit
Part Number: B400
Price: $224.95
Quantity:
The AlienBees™ B400 Flash Unit

6 f-stop power variability (5 Ws to 160 Ws)
stepless slider adjustment from full to 1/32 power
0.5 second recycle to full power
1/2000 second flash duration (t.1) at full power
internally fan cooled for heavy duty use
WYSIWYG modeling previews
standard 120 VAC, 50-60 Hz power requirements
2.5 pounds total weight
60-Day Absolute Satisfaction Guarantee
2-Year Factory Warranty

Hmm, are you just taking studio portraits of the kids? That's what lighting sets are for. Don't you really want to take pix of them doing stuff inside? You're not going to want to set up lights for that, and they may hate you for it. You may want a camera that performs better indoors, with regular house lighting.

If I were you, I'd trade up with my camera. The newer cameras have much better low light performance (higher ISO rating). See your camera compared with the newly obsolete D90 (but still available), and the recent D7000.
http://www.dpreview.com/...ikon_d7000

Another idea. Get the Nikkor 50mm f/1.8 lens. It will take excellent pix in low light on your camera.