You can stop malicious sites in many ways:
Blacklist the website on your router/modem
Place a parental control application on your child's devices
Enabling the browsers built in parental control
Blacklisting sites on your modem (xfinity: https://10.0.0.1 | AT&T: http://192.168.1.254)
Navigate to Parental Control > Manage Sites > Enable It
Click add next to blocked sites/keywords
Type in the website you want blocked & Select whether you want it always blocked or only during certain times, (Make Sure You Click Save)
You can choose devices you deem safe to exclude for these blocked sites
You can Now also add keyword restrictions Ex: if you block "sex" when they type that in on any browser it will redirect to a "SITE/KEYWORD BLOCKED" page
Blocking sites directly on the device
Most devices come with an option to enable parental controls
Androids: goto settings > search for parental controls
Iphone: goto settings > tap screen time > tap continue and select "This is my [Device]" , " this is my child's [Device]"