Is mercy toward children a cause for entering Paradise?

Yes, showing mercy to children is among the causes for entering Paradise. It was narrated from Aisha (may Allah be pleased with her): 'A poor woman came to me carrying two daughters of hers. I fed her three dates. She gave one date to each of them and was about to eat the last one when her daughters asked her for it. She then split the date she had intended to eat between them. I was impressed by her action and mentioned it to the Messenger of Allah (peace be upon him). He said: "Allah has indeed decreed Paradise for her because of it, or has saved her from the Fire because of it."' (Agreed upon).