What is the meaning of the Hadith mentioned in Fath al-Bari: "The Prophet (peace and blessings be upon him) used to command his foster mothers on Ashura, and the infants of Fatimah, and he would spit into their mouths..."?

The text of the hadith is: "He used to command his suckling infants and the suckling infants of Fatimah, so he would spit in their mouths and command their mothers not to nurse them until night." Ibn Hajar said that its chain of narration is "acceptable" (la ba's bihi), but Al-Albani declared it weak. This hadith indicates the fasting of children and is supported by the hadith of Al-Rubayyi' bint Mu'awwidh, narrated by Al-Bukhari and Muslim, in which she mentioned that she and her children used to fast 'Ashura', and if one of them cried, they would give him a toy until iftar. This indicates training children in fasting and acts of worship, even if they are not yet religiously obligated.