There are often descriptions of female nudity , but they're limited to upper torso. Female breasts are mention fairly often, but that's pretty much it (i.e. certainly not on the level of the cards from the first game). Assuming the child follows this order (starting with the short stories), there isn't much to be called explicit.
I'll try to provide some examples of what you'll face when reading, and leave the consideration to you, since I'm no expert on age ratings.
The only "sex"-like scene from the first book (The Last Wish) I can remember right now is this (from "The Voice of Reason I"):
The girl carefully climbed onto the bedclothes, and onto him, wrapping her thighs around him. Leaning forward on straining arms, she brushed his face with hair which smelt of chamomile. Determined, and as if impatient, she leant over and touched his eyelids, cheeks, lips with the tips of her breasts.
There's an implied sex scene in "The Last Wish", and an implied sex scene in "The Lesser Evil":
She put her legs on his knees. ‘Pull my boots off. A high boot is the best place to hide a knife.’
Barefoot, she got up, tore at the buckle of her belt. ‘I’m not hiding anything here, either. Or here, as you can see. Put that bloody candle out.’
Outside, in the darkness, a cat yawled.
‘Is this cambric?’
‘Of course it is, dammit. Am I a princess or not?’
And that's it for the first book. There's not much in the second book (The Sword of Destiny), either, except maybe this part which I'm not sure what to classify as (from "A Shard of Ice"):
Angry at the cold and congealed scrambled eggs which the innkeeper, distracted from feeling up the girl who worked in the kitchen, served him. Particularly annoyed that the girl was barely twelve years old and tears stood in her eyes.
And this scene:
She nestled against him then wriggled around so that she could remove her nightgown more easily. Delighting in her nakedness, as usual Geralt felt a shiver go down his spine and a tingling in his fingers as they came into contact with Yennefer's bare skin. His lips lightly touched her breasts, rounded and delicate with nipples so pale they were only apparent by their prominence. His hands got lost in the tangle of her hair, sweet with the fragrance of lilac and gooseberries.
There's this "sex scene", but that's the entire description (from "Something More":
She slid back on the cloak arranged over the moss. Geralt kissed her breasts. He felt the nipples harden and rise up under the fine fabric of her blouse. Yennefer was breathing raggedly.
Then there's this "failed sex" scene from a later book (The Tower of the Swallow):
The contact of fingers was followed by the contact of lips. [Ciri] was on the verge of forgetting the whole world, when [. . .] suddenly stopped moving. For a while, she lay patiently, reminding herself that he was wounded and the wound was probably bothering him. But he took too long. His saliva began to dry on her nipples.
And this one from the last book (The Lady of the Lake):
They fell in a pile of parchment that scattered everywhere under their weight. Geralt stuck his nose into Fringilla’s neckline. He hugged her and grabbed her knee. He rolled up her skirt to her waist knocking over several books, including Lives of the Prophets, full of mysterious illustrations, as well as De Haemorrhoidibus, an interesting, though controversial medical treatise. The witcher pushed aside volumes and pulled at the dress impatiently. Fringilla eagerly raised her hips.
Something pushed against her shoulder. She turned her head. Learning the Art of Midwifery. Quickly, so as not to tempt the devil, she looked in the other direction. The Sulphurous Hot Springs. In fact it was getting warmer. From the corner of her eye she saw an open book which rested by her head. Reflections on Inevitable Death. Even better, she thought.
The witcher struggled with her panties. She raised her hips, but this time only slightly, so that it looked like a random movement and not defiant help. She did not know him and did not know to respond. Whether he prefers that a woman knows what she wants, or does he like a woman who pretends she doesn’t know. And if he would be discouraged by panties that offered resistance.
The witcher, however, seemed to show no signs of discouragement. You could say the contrary. Seeing that it was time, Fringilla eagerly spread her legs, bringing down books and pamphlets stacked in piles, which poured over them like an avalanche. A heavy, leather bound copy of Mortgage Law painfully struck her in the ribs and the Codex Diphmaticus, adorned with brass fittings, fell on Geralt’s wrist. Geralt assessed and took advantage of the situation – placing the large tome where it was necessary. Fringilla squeaked because the fittings were cold. But only for a moment.
I think this is enough so that one can see the general tone in which the sex scenes are written (in fact, I can't think of any other scenes from the books, so sorry for spoilers :D).
As for gore - I could call it graphic, but I think it's more accurate to call it surgeon-like. Characters do cut each other's hands and other limbs off, and there are occasional spatters of blood, but I wouldn't call them shocking. There are some scenes with implied past gore, e.g. blood on the walls and witnesses' testimonies, but I don't know if those are graphic.
Those quotes are harder to find than the sex scenes, so it'll take me some time.
My verdict: I'd say +14 years is fine - they see smuttier things on the Internet, and if it's going to be adult-oriented, then it'd better be The Witcher. However, I would also advise trying one of the comics, preferably Fox Children - that one should be reasonably gore- and sexuality-free, and it's a faithful adaptation of one of the stories from the latest published book, Season of Storms.