If you possess the minimum amount of wealth (nisab) and your child was born before subah sadiq (the break of dawn) on the day of eid ul fitr, it is mandatory for you to give Sadaqah al Fitr on behalf of the newborn baby. If your child was born on the day of eid ul fitr after subah sadiq, it is not mandatory for you to give sadaqah al fitr on behalf of the newborn baby.
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دلائل:
الفتاوی الھندیۃ: (192/1، ط: دار الفکر)
ووقت الوجوب بعد طلوع الفجر الثاني من يوم الفطر۔۔۔۔۔ومن ولد أو أسلم قبله وجبت ومن ولد أو أسلم بعده لم تجب۔۔الخ
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.