Apart from more basic solutions like setting up parental controls to restrict and monitor what kids see and do online, a good next step would be to read online safety advice tailored to kids age and online activities. I'm guessing if they are very young they don't yet use social media, but perhaps they play some games online and that could already expose them to harmful content or people with bad intentions. But there's separate safety tools for gaming, too.

Internet Matters has lots of good, and free, resources on all of that for both parents and kids, for instance.