Child Benefit is tax-free, so HMRC won't regard it as contributing to your income for tax purposes. It's payable to anyone who is responsible for a child, at a rate of £20.30 per week for the oldest (or only) child and £13.40 for each subsequent child.
Child benefit doesn't count as income when other benefits are being assessed (but you should notify the relevant offices if you're in receipt of Council Tax Benefit and/or Housing Benefit):
http://www.direct.gov...sforparents/DG_073790
See here for full information:
http://www.direct.gov...ildbenefits/index.htm
This page (and, probably more importantly, the links from it) will tell you about Child Tax Credits and how much you can get:
http://www.hmrc.gov.u...y-change/new-baby.htm
Chris