I configured cloudflare family friendly dns for my child, however, it is not blocking adult contents(specifically on vk.com, where I caught my child going there to watch some bad stuff despite configured family friendly dns server), it is quite unimaginable and shocking why cloudflare isn’t blocking adult contents on such a popular website, and not living up to its promises.
What feature, service or problem is this related to?
1.1.1.3 is a DNS-based service so blocks sites by name, it cannot block based on actual content passing to you.
As vk.com appears to be a social media site, it is like Facebook - a site that is not classified as adult itself, but obviously users could post adult content to and is up to the site to censor content. You will need an alternate solution to either block the site yourself (such as at your router, or using approproate software, or a Cloudflare Zero Trust Gateway policy) or software that can analyse content as it arrives.