Cheapest software of all is Open DNS. Also, siting the computer for the kids' use in a public place within the family environment will be better than any number of Net Narnies or Spies.
Some people, myself included, would suggest that the home computer is not the only one your child will have an opportunity to access "adult" or disturbing material on. Whaddya' do about that?
Is blocking software the right approach at all? Could you defeat such software, or know someone who knows a way to defeat it (a handy, online message board, for example)? I'm certain I could find a way if I were determined enough.
This brings up the less compact alternative (than a 'pay-fee, click-setup' software approach) of finding time for discussions with your child, sharing and listening to them, educating yourself and them about online risks.
Our Internet access is increasingly being censored/monitored by our governments. Training your children to accept restrictions imposed from above at an early age steps into the same pattern.