I'm on the fence about 'fresh mouth' - I think if I put my literature teacher hat on, I'd say, there are many connotations - yes, there is a sexual connotation, but also a connotation with youth, and a connotation of being cheeky or quick-witted. As an image it makes me think of really shiny lip gloss, so there's that as well. Sexuality is one of the connotations but I think it's reductive to say that's all it could possibly be, and just silly to call it a safeguarding issue.
Writers do not have to consider safeguarding concerns in their writing. If they did, there would be no more crime fiction, or 'true crime' for that matter. KC's memoir does not put anyone at risk of harm.