Si l’on regarde la vidéo on voit que les enfants sont jeunes, trop petits pour faire de l’ironie ou du sarcasme. Ça a l’air sincère (ils sont à l’âge où l’on apprend à ne pas se donner des claques).

Dans cette situation la principale "clue" pour répondre c’est la vidéo, le ton et l’âge des enfants. Cute !

The actual formulation of the question follows a pattern used often in informal conversational French. Indeed, this type of question can be used to make innuendos of various sorts that will be recognized as such through the particular intonation; however, the intonation itself gives no clues as to what is precisely meant. The context will be what allows one to determine more definitely whether the question is sarcastic or a rhetorical question, or just a plain question; there’ll be cases when nothing is certain, as well.

In this present situation there is not the least possibility of a sarcastic intent, and much less still that of a rhetorical question being put to the president. The reason is simply that the children are much two young to have assimilated those ideas. It will be noticed, if one listens to the video, that the child has difficulties in the articulation of the question; he pronounces the word "claque" twice as if unsure of his words and then "que" is barely audible (maybe due to low-fidelity recording though). As he still has difficulties to express himself fluently, it is even more unlikely that he could have at his disposal the means of using the language for insinuation.
Even if, from the point of view of what the question implies plainly, it is a question that does not correspond to the child’s age, it has nevertheless a simple, although rather unclear signification. What is he asking in fact?  Whether the president recovered? That is not a relevant notion; what consciousness of any medical consequence of the slap, whether bodily or psychological, could any adult have had? None, nobody in their right mind would have had such thoughts. Yet, the asking "ça va …?" cannot be taken as being on any other level ; would one suspect that the child could have been motivated by matters of legality, popularity, or matters having to do with the dignity associated to the function of someone in high office? That is even more far-fetched. No, there is not much common sense in the asking of this question, and it’ll have to be dismissed as one more instance of the inchoate fancies a child’s mind is capable of.

The president does not emit any judgment as to the reprehensibility of the act of slapping him or of slapping anybody else; his reply is strictly on the level of the plainest meaning that can be found for the child’s words, but not specifically to the question, as the point in that question is not very explicit (it has to be inferred from the words); he answers "Ah bè oui …C’est pas agréable, hein… c’est pas bien.". He is not replying according to what has been asked, that is to the question of how he lives with the consequences of what the slap has done to him—to give a more specific wording to the question—, but instead somewhat besides the point, which he does by the formulation of an appreciation of what that sort of experience entails for someone’s who has been subjected to it; it is the least you can say about it and sure to be true; "people can’t be feeling good being treated that way" is the meaning that can be given to his words.

To give an explicit answer to the last question, the difference between plain and sarcastic question is made only by means of the context that gives rise to the question and by the intonation used in that latter. Moreover, the intonation can be normal and sarcasm still be the intent (there are certain very insidious possibilities as for instance that of a very affectionate tone of enunciation); then you can rely only on the context.