Initially Hiranyakashipu did throw Prahalad into the fire, but that was of no use, then after repeated attempts, His sister Holika wanted to help Hiranyakashipu in killing his child, Holika has a fire shielding shawl which she wrapped around herself and made Prahalad sit on her lap and when the fire was breaking Screaming voices were heard( the demons thought it was Prahalad as (children used to have smooth voices like women),when the fire was done Prahalad was seen praying Narayana and was covered with the shawl whereas Holika was burnt to death, So I hope you got it now, yep Demons are not soo foolish they have tried sooo many times but none worked
P.S- this was the story I heard from my elders and wikipedia mentions it https://en.wikipedia.org/wiki/Holika#:~:text=Holika%20had%20a%20special%20cloak,death%2C%20Prahlad%20came%20out%20unharmed.
But I don't know the book that mentions regarding Holika, Sriad Bhagavatham doesn't mention anything regarding Holika in Prahalada Charitra