I'm not sure if this answer fits the title or the part about "heat" or the missing word in the last sentence (if those are even clues), but I'd guess it's
Here is my reasoning:
I am born from the death of my mother
The bottom is "born" and filled as the top is drained, creating the new from the death of the old
and I have millions of secrets to hide.
Millions of grains of sand in an hourglass
Like me there are plenty of others
There are many different types of timepieces, also many different hourglasses
but we are all empty inside.
There is always at least enough space for one half of the glass to be empty, and the "child" we're referring to always starts empty
Maybe you know, I'm never in a hurry
Ever stare at an hourglass and try to will the grains to fall faster?
due to my origin sublime.
Hourglasses measure time, a very sublime concept
At the same time, I am always hungry,
not for knowledge,
but for time.
The grains constantly fall through into the bottom until the top is drained, but no matter how many times you flip the glass over, it will still be hungry for more