Instead of banning internet altogether which will make young children more curious and inquisitive, set time limits and discuss openly why are you restricting internet usage in the first place.
Try and keep the computer at a central location. Talk to them with a friendly approach if you see them spending more time alone than usual and secretive about their online activity. Encourage educational games and learning activities on the internet that are fun and informative at the same time. Also, ask your children to come to you if they have any questions about their body or personality rather than looking for answers on their own.