The first step in keeping you children safe is to learn what dangers are out there and how to prevent access to such materials. The next is to get involved with your children as they surf the net. You must lay down rules for their access time and establish when they can be online, for how long, and where they can visit. Emphasize to them the importance of not breaking the rules or exploring beyond approved areas and the consequences if they are broken. Since you usually cannot see the face of the person you are communicating with, you cannot tell what their true intentions are.
Most people are honest and have trustworthy motivations.
But there are always those whose motivation and actions are pure evil. It's a good idea to place the computer in a family area vs. enclosed in the child's room. This will serve to deter children from exploring where they shouldn't be and help you in periodically monitoring what's on the screen. Keep track of the time your child spends online as it is very easy to lose track of time. You might want to set a clock or timer nearby.