Virtually all video games and social sites and programs online require an account and login.  Be involved in your childrens' account setups. Almost every account, whether it's video games, Google, social media, etc, has Content Filtering and/or Parental Control settings.  The best thing you can do as a parent is to setup your child's account as a "child account" under your account.
The next best thing, if setting an account as a child account is not feasible, is to set your child's approximate age in their account and setup content filtering and privacy options in their accounts. Many websites and programs can be made immensely safer by simply changing account settings.

Here are some key items to look for and setup:

• Privacy settings.
  Make accounts not searchable by public.
  "Who can message me" - friends only
  "Who can send friend requests" - friends of friends only
  "Who can see my profile information" - friends only
  "Who can see my real name" - nobody
  "Who can see my email address" - nobody

• Content Settings
  Not all sites or apps have age specific content settings, but you should always look for these settings and set what you feel is appropriate for your child.

Depending on your child's age, you should put a lot of thought into allowing them on any social media or instant messaging platforms. Some sites are simply not appropriate for kids, like Twitter.
Some sites have kid-only versions, like Facebook and YouTube. Some sites, Facebook for example, have a "kid mode" where parents have to approve every contact request. Microsoft Family Safety has a request system where a child can send a request for access to websites that Microsoft has flagged as questionable. 

The settings and configuration options are very different for every platform, but they usually have similar options.
Parents should seek out the privacy and content settings for EVERY account their child has.
Parents should evaluate their child's maturity level, your personal beliefs, and choose the child settings level of a site or program accordingly. Different kids may need different settings and different beliefs may stipulate different settings as well.

There can be a balance between complete restriction and full access that both parents and children can be happy with!

Almost every device - computer, phones, tablets have settings for parental controls and content filtering. 

• Apple makes these settings fairly easy to get to on iPhones/iPads and Mac computers.

•  For Windows computers, you'll need to have your child signed in with their Microsoft account, which is where you'd set the safety and privacy settings.

• Android devices can be configured both on the device, as well as the child's Google account settings. Alternatively, you can install Microsoft Launcher and Microsoft Edge and take full advantage of Microsoft Family Safety settings on an Android device.

• Xbox also will use settings from the Microsoft account and Microsoft Family Safety settings.

• Playstation has options to create a child account. The child account must be linked to a parent account. The options are not as varied or granular as many other services, but it's still a valuable resource

One of the worst things to do is to let children use parents' devices while signed in as the parent all the time. At the minimum, most devices, even iPhones, have options to switch user or switch profile.

If your child has their own devices, they should be signed in with the accounts you have setup or configured for them.
There will often be multiple accounts to configure in device settings and multiple apps.

Almost all devices have family/child settings guides on the manufacturer's website. The setup is usually pretty straightforward. 

Many home routers ('wi-fi boxes') also have built in content filtering, as well as useful settings like time restrictions, etc. Consult your router manual or contact your Internet Provider if using an ISP issued router.