When very young children start using a computer, they should be supervised closely by a parent or caregiver.
Parents may wish to choose the websites their kids visit early on — and not let them leave those sites on their own. If little kids aren't supervised online, they may stumble onto sites that could scare or confuse them.
When you're comfortable that your young children are ready to explore on their own, it's still important to stay in close touch while they go from site to site. You may want to restrict access to sites that you have visited and know to be appropriate — at least in terms of their educational or entertainment value.