How to differentiate the work for children from the work for adults? Let's face it. I don't want the adult material to get to the kids either and I don't blame the other adults for wanting to protect their kids.
There is Uranian Fiction which is for adults and there is some for children. As with movies, there are adults who will enjoy the stories for kids...I had to think about a nice metaphor for you...I don't think the adults would find either the Jimmy Bodeilo stories or 'What is Trillium?' or 'Sharbenau the Squirrel' as uncomfortable as ....children's clothing or walkers or bibs or My Pretty Pony....but would find it to be like with Universal rated films.
The novel An Adventure in Indianapolis and the story The Future of Engineering are written for adults, not for kids....although if a kid gets hold of F of E, it is not the most dangerous. I would avoid giving them An Adventure in Indianapolis. I don't really want them to get their hands on anything I wrote without them [people under 18 years] in mind. I feel the work is 'too inconsiderate' and may be dangerous - depending on the young person.
In general, I will endeavor to be clear about what age group a work is for, having the best interests of everyone in mind. In some cases, publishers will help - Writings from the Heart: Stories and Poems from Around the World is obviously for young people.
When in doubt, I urge adults to check it over. Again, I like both kids and adults, and just don't really want to stick kids with material written without taking their unique needs into account. That's all.
As it says at the website: "If it is a Jimmy Bodeilo story, you know it is for kids." Those stories are not for babies, they are for the 8 - 12 year old crowd, but will please some older people and should be as safe as 'nontoxic paste' for younger children.