Hm, I'm actually doing a story like this so I've run into the problem of knowing it will attract younger people or people wanting romance from it because 1. The fandom 2. Art style. (Mine is a fancomic that's why I mention the fandom)
My story is pretty heavy on mature subjects (as in for adult subjects, not necessarily sexual elements) but since my artworks are 'cute', it can give off the wrong impression, thus, I make sure to warn people in the first chapter about it.
I'm currently remaking it with better art but I remember in my old version, I deliberately started really crude just to scare off people who expected it to be all happy and innocent... Though, this time around, since the prologue is mostly on the 'wholesome' side, I first added the warning pages and then started with a mildly serious thing happening.
I'm personally not fond of realistic art styles (it's cool and all but it doesn't appeal to me when reading. Individual artworks I can ofc appreciate ), so it's not like I draw the way I do deliberately wanting to create the contrast, though in my opinion, the cute art does help for example emphasize the innocence of some of the characters. But I do try to make it creepy when it needs to be 
I'm one of those that like stories that appear cute but are serious and not the other way around (though I guess it would depend on the art style, however, it's rare that I'm looking for stories that are calm for me to run into something creepy but calm)
I've read many things and watched from many genres, so, it may not appear to be but I'm quite flexible as long as the story is good. I just can't put up with mangas that look too much like real humans (kinda like western comic style) or when there's too many details in the panels that I have a hard time distinguishing what's going on. Like for example 'Berserk', I know it's a pretty good manga and all, but the amount of detail on everything isn't my cup of tea (even if I can appreciate the talent and the awesome art it has)
Soo, back to what you stated, I think my major concern is the story being recommended to children when it's not supposed to be read by sensitive audiences xD So I believe the best thing to do as courtesy is to warn them at least at the beginning even if you don't want to warn every chapter.
Oh though, I want to point out too that I can't stand exaggeratedly cute mixed with violence like for example 'Happy tree friends' I could never stomach that one. It ends being too creepy for me (ironic but yeah xD)
Guess balance is everything even if its dissonant haha. (Though well, there will always be people that like stuff like Happy tree friends)
But yeaaah that's what I think XD I love psychological, dramatic, thriller stories with art styles that are on the cute side and are 'something nice to look at'.