Our friends at Protect Young Eyes wrote an extensive article that will help you prevent your child from encountering pornography on any device (for free).
https://protectyoungeyes.com/how-to-block-porn-on-any-device-for-free/
Blocking pornography is great, but blocking alone doesn’t prepare a child for what to do when he/she sees it for the first time. Don’t forget that it’s equally important to discuss this topic openly with your child.
https://protectyoungeyes.com/how-to-talk-to-a-5-year-old-about-porn/