The first thing is to talk to them about why they need to be careful. It's very important that they know, if they do encounter a strange or scary situation, they can and should come straight to you and tell you, without worrying that you may get angry or overreact.
It's really best if young children don't have a computer in their room, or aren't able to spend time accessing the internet on phones etc. They really don't need it and it's bad for them to spend such long hours indoors looking at a screen, quite apart from the online dangers. But obviously your child at some point will start using the internet when you aren't around, so they need to be aware. It's also advisable to use a child-friendly browser so that they are protected from inappropriate content while they are still too young to deal with it.

This UK site contains very sensible and clear advice.