Стоит мне открыть коробку с пирожным, как дети сразу спрашивают, есть ли там пончик с глазурью.

This is a fixed Russian pattern meaning:

• As soon as I ... , ...
• The moment I ... , ...
• sometimes literally, It only takes me to ... for ... to happen

So Стоит мне открыть коробку..., как дети сразу спрашивают... means that the children ask immediately whenever that opening happens.

This pattern is very common in Russian for something that happens right away after something else.

Because this construction uses the dative case for the person involved:

• Стоит мне открыть...
• Стоит ему сказать...
• Стоит нам уйти...

So мне is not the subject in the usual nominative way. In this pattern, Russian says something like:

• It only takes for me to open...

That is why you get мне, not я.

Открыть is perfective, and here it fits because the sentence refers to a single completed trigger event:

• I open the box → immediately the children ask

The focus is on the moment the action happens and is completed. That is exactly where perfective is natural.

If you used открывать, it would sound more like an ongoing or repeated process, which is less suitable in this specific structure.

Because открыть takes a direct object, and direct objects are usually in the accusative case.

• nominative: коробка
• accusative: коробку

So:

• открыть коробку = to open the box

That is standard verb + direct object grammar.

Пирожным is the instrumental singular of пирожное.

The preposition с meaning with normally takes the instrumental:

• с пирожным = with a pastry / with a dessert

So коробка с пирожным literally means a box with a pastry in it.

A small note: depending on the exact situation, some speakers might also say с пирожными if they mean a box containing multiple pastries. But grammatically, с пирожным is completely normal.

Russian often uses the present tense to describe something that happens regularly, typically, or whenever the situation occurs.

So here:

• дети сразу спрашивают = the children immediately ask

This does not mean they are asking right this second only. It means this is what they usually do every time that situation happens.

In English, we also do this:

• Whenever I open the box, the kids ask...

So the Russian present tense here expresses a habitual/repeated action.

Russian often leaves out subject pronouns when the meaning is already clear from context or from the verb form.

• спрашивают already tells you it is they ask
• the noun дети is also already there, so они would be unnecessary

Russian generally uses pronouns less often than English does.

Ли is used to form an indirect yes/no question.

So:

• есть ли там пончик...? = whether there is a doughnut there / if there is a doughnut there

This is not the same as если, which means if in the sense of condition:

• если будет время = if there is time

But here the meaning is not conditional. The children are asking a question, so Russian uses ли.

A very common pattern is:

• Я не знаю, придёт ли он. = I don’t know whether he will come.
• Они спрашивают, есть ли там... = They ask whether there is...

In Russian, ли usually comes after the word that is being questioned or focused.

Here the question is about existence:

• есть ли там пончик = is there a doughnut there?

So ли follows есть.

Compare:

• Он спросил, придёшь ли ты. = He asked whether you would come.
• Я не знаю, дома ли она. = I don’t know whether she is at home.

The placement of ли is one of those things that becomes more natural with exposure, but the key idea is: it usually goes right after the focused element.

Because the sentence has two linked clauses in the pattern:

• Стоит мне открыть..., как дети...

This construction normally uses a comma between the two parts.

So the structure is:

1. trigger event: Стоит мне открыть коробку с пирожным
2. immediate result: как дети сразу спрашивают...

The comma marks that division.

Глазурью is the instrumental singular of глазурь.

Again, the preposition с meaning with takes the instrumental:

• пончик с глазурью = a doughnut with glaze / icing

This is a very normal way to describe food in Russian.

You may also sometimes see similar food descriptions like:

• чай с лимоном = tea with lemon
• булочка с корицей = a bun with cinnamon

So с глазурью is just the regular with + instrumental pattern.

No. Here как does not mean how.

In this construction, как works as part of the pattern Стоит..., как..., where it means something like:

• and then
• when
• than immediately

So in this sentence it helps express the immediate result:

• As soon as I open the box, the children ask...

This is a special usage that learners usually need to memorize as part of the whole expression.