You don't stop educating your children once they've learnt their phonics. They need to move up to understanding the meaning of what they're reading. In the same way, once your child is online and you're filtering and monitoring in the right way for their age, there's still a job to do. Here's a useful breakdown of what it means to be digitally literate, with some good news for parents.
You can't shield your child from every risk in the online world, any more than you can offline. So how do you help them to be digitally literate (what does that even mean?) And what kind of parenting approach is most likely to help them stay safe?