“Let Alone” in a sentence

The usage is idiomatic rather than rigidly grammatical. do is not needed.

Let alone = used to emphasize that something is more impossible than another thing:

You couldn't trust her to look after your dog, let alone your child

means “You would not trust her to look after your dog; it is even less likely that you would let her look after your child.”

Cambridge dictionary

Cambridge dictionary uses impossible but your example is better understood as improbable. “Few adults know Python; is is even less probable that children know it.” (This is the meaning of the sentence, regardless of any discussion of its truth)

The precise rules of the let alone construction are specified in great detail in
Fillmore, Kay, and O'Connor's classic 1988 article

"Regularity and idiomaticity in grammatical constructions: the case of let alone"
Language, Vol. 64, No.3 (1988), pp 501-538

In particular, in this case, there should be no do in the let alone phrase. There is no fronting, negation, or inversion involved in this construction, and thus no need for Do-Support.

So the correct version is

• Python is something that not all adults know, let alone the children.