Why was the word "الذين" (those who) used with "الطفل" (the child) in the verse: (Or the child who has not yet appeared on the nakedness of women)?

The term "al-tifl" (child) is applied to both singular and plural. It can be for one or for many, similar to "al-junub" (one who is in a state of major ritual impurity). Its occurrence in the Holy Quran with the plural form (alladheen - those who) confirms that it is used for both meanings.