I want to know that whether the death of Janab e Sakina(-the 4 year old baby child of Hazrat Hussain) and the death of Prophet Muhammed, both of these person's death were martyrdom or just normal natural death? please reply with reason also for the answer.

Bibi Sakina died a natural death.
The Prophet ﷺ was poisoned. It is mentioned in Tafsir Qummi and Tafsir Ayyashi in the commentary of 3:142