Four of the children who died were minors that included two girls.
Police said the incident took place at Luxmi Nagar locality here yesterday night.
Police identified the deceased as Abhimanyu (21), his two minor brothers Arshu (15), Anurag (12) and two sisters Anu (18) and Archna (8).
SSP Sandeep Sharma said Abhimanyu took the extreme step as the family was living in extreme poverty.
He bought burgers from the market and allegedly laced it with some poisonous substance and asked all the deceased to eat it. He himself consumed it to end his life, Sharma said.
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Abhimanyu had also given the burger to his 7-year-old sister Aarti, who has been admitted to a hospital.
In her statement, Aarti said that she vomited soon after consuming the burger.
The deceased Abhimanyu left a suicide note, saying, "Papa, I miss you and am sorry for it as we were a burden on the family", police said.
The father of the deceased Kishore Thakur, who is a barber, was not in the city when the incident occurred.