As your kids get older, there's more concern they might navigate to a website that's inappropriate for them. No solution is 100% foolproof, but here are a few ways to lockdown the devices they use to restrict their surfing to age-appropriate sites.
Apple:
Go into Settings > General and look for the option called Restrictions.
In here you'll find all types of things you can control. With just a few taps you can turn off entire features like the camera, Siri, or even Safari. There are also toggles to keep kids from installing new apps, or making in-app purchases. You can also restrict content to certain ratings, just keep in mind these settings pertain to Apple content and not the stuff your kids are streaming inside Netflix or Amazon Prime.
Android:
Android is a bit more complicated when it comes to parental controls since there are so many different versions of the operating system. There are big differences depending on the maker of your device. But in general, a good place to start is to look in your settings and see if you can add a second user. Then, you can go into the Google Play store, and under settings you'll find lots of options for restricting content by ratings.
If you have a Samsung phone they have a fantastic app you can download called Kids Mode. Once you activate Kids Mode you have control over everything your child can do on the device. You can allow access to certain apps, contacts, videos and even the camera.
Netflix:
Netflix, has a plethora of parental controls built in to make your life easier. Just sign into your master account and then go into Your Account and look for Manage Profiles. Here you'll get the option to restrict content based on four options including Little Kids, Older Kids, Teens and Adults. The changes you make here will be reflected across all of your Netflix devices.