Those VPN services don’t as rule offer “safe” controls for a parent, they simply mask your IP address with theirs.
If you want to ensure your children don’t come across dodgy material, then I believe some DNS services have family offerings, look up cloudflare, OpenDNS etc. it will block some stuff.
I think you are not clear on what protection VPNs offer it’s not what you think, ignore their claims.
Depending on your knowledge levels and pain thresholds, if you get pfsense running you can add pfblocker across your network, it will cut out a lot of crap, porn, gambling etc. the beauty of this solution is that regardless of what your children do it will be difficult for them to circumvent.
Encryption is between two points, so basically with VPN providers you are paying for the ISP not to see what you are doing.
If you are down the pub and want to use their free wifi, then with pfsense and OpenVPN you can create a secure/encrypted connection for free back to your home network and out to the internet, however, in this case (depending on your knowledge) using a paid for VPN might be easier to set up.
If you are not up to the effort of pfsense, then you can easily buy an Asus router (which allows OpenVPN clients) and set up a paid for VPN on it for the whole network, if you believe they block out all the crap then it can be applied across the network.