Mi madre dice que es mejor no usar lejía hasta que los niños salgan de la cocina.

After decir in Spanish, you normally use que to introduce what someone says:

• Mi madre dice que... = My mother says that...

In English, that is often optional, but in Spanish que is usually kept.

Because es mejor is followed here by an infinitive phrase:

• es mejor no usar lejía = it’s better not to use bleach

Spanish often uses the infinitive after expressions like:

• es mejor
• es importante
• es necesario

This is especially common when the advice is general, not directed at one specific person.

A more personal version would be:

• Es mejor que no uses lejía = It’s better that you don’t use bleach

Because no is negating the infinitive usar:

• no usar = not to use

So es mejor no usar lejía literally means it is better not to use bleach.

Here lejía is being used in a general, uncountable sense, like bleach in English. Spanish often omits the article in that situation:

• usar lejía = to use bleach (in general)

If you said usar la lejía, it would sound more like you mean a specific bleach already known in the conversation.

Yes. In Spain, lejía commonly means bleach, especially the cleaning product.

In other Spanish-speaking countries, people may more often say things like:

• cloro
• blanqueador

So lejía is very natural for Spanish from Spain.

Because after hasta que, Spanish uses the subjunctive when the action has not happened yet and is seen as future or pending.

Here the children have not left the kitchen yet, so Spanish says:

• hasta que los niños salgan de la cocina

If it were a habitual or already-known action, the indicative could be used in another context, but not here.

Salgan is the present subjunctive, third person plural, of salir.

The verb salir is irregular in the present subjunctive:

• yo salga
• tú salgas
• él/ella salga
• nosotros salgamos
• vosotros salgáis
• ellos/ellas salgan

So los niños salgan means the children leave / go out in a subjunctive context.

Because the verb salir normally takes de to mean to leave or to go out of a place:

• salir de la cocina = to leave the kitchen / go out of the kitchen

So de is required by the verb here.

Los niños refers to specific children, or to the children understood from the situation.

Spanish often uses the definite article where English may not emphasize it as much. Here it sounds natural because the speaker probably means particular children in the house or in the kitchen.

Just niños would sound less natural in this sentence.

Yes, you could, but it is less neutral.

• hasta que los niños salgan de la cocina is the normal, everyday way
• hasta que los niños hayan salido de la cocina adds a stronger sense of until they have left

In most ordinary situations, salgan is the most natural choice.

Because each verb is doing a different job:

• dice is present because the mother is saying this now
• es is present because the statement it is better is true now
• salgan is present subjunctive because it refers to a future action after hasta que

So even though the children will leave later, Spanish still uses the present subjunctive here, not a future tense.

Yes. A very common alternative would be:

• Mi madre dice que es mejor que no uses lejía hasta que los niños salgan de la cocina.

That means something like:

• My mother says it’s better that you don’t use bleach until the children leave the kitchen.

The original sentence sounds more general and impersonal, while this version sounds more directly aimed at you.