If poetry is inside your search span, I’d suggest reading Jacques Prévert‘s poems. No need to be an excellent french reader to read most of them. But it definitely needed a true genius to write them in the first place.

Now two examples to match Nikana’s expectations (I’m kidding but you were right of course).

First, this one moment of sheer perfection and simplicity :

Premier jour

Des draps blancs dans une armoire
Des draps rouges dans un lit
Un enfant dans sa mère
Sa mère dans les douleurs
Le père dans le couloir
Le couloir dans la maison
La maison dans la ville
La ville dans la nuit
La mort dans un cri
Et l’enfant dans la vie.

Or else, this one, that I find amazing too :

Dimanche

Entre les rangées d’arbres de l’avenue des Gobelins
Une statue de marbre me conduit par la main
Aujourd’hui c’est dimanche les cinémas sont pleins
Les oiseaux dans les branches regardent les humains
Et la statue m’embrasse mais personne ne nous voit
Sauf un enfant aveugle qui nous montre du doigt.

The wiki page and this website may help you to discover the man and his writings further.

Interesting question.

I would say that lot of children books are well written, simple to read yet enjoyable for adults. From illustrated books for the youngest to short novels. Comic books, if you are into it, might be interesting too.

Out of the blue, I would say you might enjoy « Les contes du chat perché » from Marcel Aymé. Of course, other « fairy tales » are good too, although Grimm’s one are not natively French (but have classical translations). You still have the Charles Perrault tales… And works like « La Belle et la Bête ».

On a more contemporary side, I enjoyed « Verte » by Marie Desplechin, a modern sorceress story.

In the “children’s books that can also be enjoyed by adults” department, I would cite the stories of Le Petit Nicolas by Sempé and Goscinny, and also the books of Pierre Gripari (Contes de la rue Broca and similar stories).

You should also have a look at the books which are actually used to teach French children. “Le petite prince” is listed in there among many other books.

I would also suggest the Fables From Jean de la Fontaine: see here and here.

I would recommend “Les Contes du chat perché”, by Marcel Aymé.