OpenDNS has some really good filters; and if you have your router or device(s) set to use OpenDNS for DNS servers you'll have very little to worry about. Although you do not need an account set up to use their service and filters, I have one so that even when I travel I have software that updates my location (IP address, specifically) so that OpenDNS still "knows" it's me logging in from [some remote location] instead of home and applies those filters.
https://www.opendns.com/
Be mindful, though, of taking your device(s) off of your property (and therefore off of your LAN) because then they would begin using whatever DNS servers are used by [who/ whatever is giving you internet access] and that likely does not have parental controls in place. Setting your devices to use OpenDNS instead of just your router mitigates this, but it a slightly greater pain in the butt to set up initially.
PM or post here if you have questions about how to set either your router or device(s) to use OpenDNS's servers.
Aside from that, setting the basic restrictions at Google and YouTube are a good idea. I am pretty sure that both the Google Play and Apple App stores have parental restrictions. If you dig into iOS's settings you can access some pretty nice restrictions as well.
Nothing ever, however, replaces direct parental involvement. None of these technologies can be or claim to be parental proxies!
And in addition to myriad other reasons, Linux-based OS's are going to be much safer, technologically, than others (I do prefer iOS's built-in restrictions over Android's; although the Android ecosystem offers many more installable options).