Rather than DNS consider an existing solution that just lacks laws to require server operators to add it and web clients to look for it. That is RTA headers. [1] Adding a header is trivial and clients looking for it is an afternoon of coding from an intern. As a bonus there is no privacy leaks unlike third party adult verification sites. The onus would be on the parents to enable it. Teens will always get around it given they watch porn and pirated movies together from within G and PG rated video games and that will always be a thing but it could help small children.
[1] - https://www.rtalabel.org/index.php?content=howtofaq#single