Learn Hardcore Norwegian: Det som gjør meg mest stolt, er at barna spør etter mer resirkulering. - What makes me most proud is that the children ask for more recycling.

Det som literally corresponds to “that which / what” in English.
The structure Det som + verb is often used to highlight or focus something, like English “what…”:

Det som gjør meg mest stolt, er … = What makes me most proud is …

You cannot drop som here; det is a dummy pronoun, and som is the relative pronoun linking to the verb gjør. If you only said Det gjør meg mest stolt, at barna …, it would sound clumsy and less natural.

Why is there a comma before er in Det som gjør meg mest stolt, er at …?

Norwegian usually separates clauses with a comma.
Here, Det som gjør meg mest stolt is a subordinate clause functioning as the subject of the main clause er at barna spør etter mer resirkulering. A comma is commonly placed between a long subject clause and the main clause:

At barna spør etter mer resirkulering, gjør meg stolt.
Det som gjør meg mest stolt, er at barna spør etter mer resirkulering.

You may also see it written without the comma in some modern texts, but with the comma is standard and clear.

Why is gjør used with stolt? How does gjør meg stolt work?

The verb å gjøre can mean “to make (someone) [adjective]”:

Dette gjør meg glad. = This makes me happy.
Det gjør meg stolt. = It makes me proud.

So gjør meg mest stolt = “makes me the most proud”.
Another possibility would be får meg til å føle meg stolt, but gjør meg stolt is shorter and more natural.

Why is it meg and not jeg in gjør meg mest stolt?

Jeg is the subject form (I), and meg is the object form (me).
Here, jeg is not doing the action; I am the one being affected by what det som does:

Jeg gjør noe. = I do something. (subject)
Noe gjør meg stolt. = Something makes me proud. (object)

So you must use meg after gjør in this pattern.

What exactly does mest do in gjør meg mest stolt? Could I put it somewhere else?

Mest is the superlative form of mye and veldig, and here it works like “most” modifying the adjective stolt:

stolt = proud
mer stolt = more proud
mest stolt = most proud

The natural word order is gjør meg mest stolt.
Forms like Det som mest gjør meg stolt or Det som gjør meg stolt mest sound odd or poetic at best in everyday Norwegian.

Why is it barna and not something like barnene for “the children”?

Barn is a neuter noun with an irregular definite plural:

et barn = a child
barn = children
barna = the children

You never say barnene in standard Norwegian; barna is the only correct definite plural form.

What is the difference between spør etter, spør om, and ber om?

spør om = ask about (information / topic)
- Barna spør om resirkulering. = The children ask about recycling.
spør etter = ask for (requesting something to get or have)
- Barna spør etter mer resirkulering. = The children ask for more recycling.
ber om = ask for (often more direct, like making a request)
- Barna ber om mer resirkulering. = The children request more recycling.

In your sentence, spør etter emphasizes that the children are asking for more of something, not just asking for information.

What kind of word is resirkulering, and why is it mer resirkulering without an article?

Resirkulering is a noun derived from the verb å resirkulere. It is usually treated as an uncountable mass noun, similar to recycling in English:

resirkulering = recycling (in general)
mer resirkulering = more recycling

Because it is used in a general, uncountable sense, you do not need an article:

mer resirkulering (not en resirkulering in this meaning)

Could I say Det som gjør meg mest stolt er barna som spør etter mer resirkulering instead?

Yes, that is grammatical, but the meaning shifts slightly.

Det som gjør meg mest stolt, er at barna spør etter mer resirkulering.
Focuses on the fact that they ask for more recycling.
Det som gjør meg mest stolt, er barna som spør etter mer resirkulering.
Focuses more on the children (those who ask for more recycling) as a group.

Both work, but the original stresses the situation / fact more than the identity of the children.

Can at be left out, like in English “I’m proud the children ask for more recycling”?

No, at cannot be dropped here. In Norwegian you normally keep at in that-clauses:

… er at barna spør etter mer resirkulering. = … is that the children ask for more recycling.

English often omits that, but Norwegian does not in this type of clause. Saying … er barna spør etter mer resirkulering would be incorrect.

Is there another natural way to say the sentence in Norwegian?

Yes, for example:

Jeg er mest stolt av at barna spør etter mer resirkulering.
= I am most proud that the children ask for more recycling.

This version avoids the det som … er … structure and uses å være stolt av (to be proud of), which is also very common.