Is the fast valid for one who swallowed a tiny amount, about the size of a drop, with a sweet taste, that was on the face of her child whom she kissed, while feeling that it was a whisper (waswas) and she ventured to swallow it?

The fast of one who made a mistake in the intention to break the fast is valid, and their fast is not invalidated; this is based on the Almighty's saying: "Our Lord, do not take us to task if we forget or make a mistake," and Sheikh Al-Islam Ibn Taymiyyah's statement that for every matter that overwhelms the fasting person, there is no need for them to make up the fast or anything else.