From Sahih Bukhari (Hadith 1960):
Narrated Ar-Rubai' bint Mu'awwidh:"The Prophet (ﷺ) sent a messenger to the village of the Ansar in the morning of the day of 'Ashura' (10th of Muharram) to announce: 'Whoever has eaten something should not eat but complete the fast, and whoever is observing the fast should complete it.' "She further said, "Since then we used to fast on that day regularly and also make our boys fast. We used to make toys of wool for the boys and if anyone of them cried for food, he was given those toys till it was the time of the breaking of the fast
Reference: Bukhari 1960