Tu étais en train de remplir la glacière quand les enfants ont demandé où étaient les serviettes.

Both are possible, but they are slightly different in feel.

• Tu étais en train de remplir = you were in the middle of filling
• Tu remplissais = you were filling / used to fill

The structure être en train de + infinitive strongly emphasizes that the action was actively happening at that moment. It is often used when another action interrupts it:

• Tu étais en train de remplir la glacière quand...

So here it highlights the ongoing action more clearly than Tu remplissais.

Étais is the imparfait form of être for tu.

The full structure is:

• tu étais = you were

It is used because the sentence describes an action that was ongoing in the past:

• Tu étais en train de remplir... = You were filling... / You were in the middle of filling...

So étais is not the main action of filling; it is part of the expression être en train de.

Because en train de must be followed by an infinitive.

Pattern:

• être en train de + infinitive

Examples:

• Je suis en train de manger = I am eating
• Nous étions en train de partir = We were leaving
• Tu étais en train de remplir = You were filling

So remplir stays in the infinitive because it comes after de in this fixed expression.

Glacière is a feminine noun, so it takes la.

• la glacière = the cooler / the ice chest

It refers to the container used to keep drinks or food cold, especially for a picnic, beach trip, or road trip.

Examples:

• mettre les boissons dans la glacière = put the drinks in the cooler

This is a very common French pattern.

• imparfait describes a background action, ongoing situation, or something in progress
• passé composé describes a completed event or something that happened at a specific moment

In this sentence:

• Tu étais en train de remplir la glacière = ongoing background action
• les enfants ont demandé = a specific event happened during that action

So the idea is:

• You were filling the cooler when the children asked...

This contrast between imparfait and passé composé is one of the most important tense patterns in French.

Yes, quand here means when.

It introduces the moment at which the second action happened:

• Tu étais en train de remplir la glacière quand les enfants ont demandé...
• You were filling the cooler when the children asked...

So quand connects:

1. the ongoing action
2. the interrupting or later event

Because the asking is treated as a specific completed event.

• ont demandé = asked
• demandaient = were asking / used to ask

In this sentence, the children’s question happens at a particular moment while the other action is in progress, so passé composé fits better:

• Tu étais en train de remplir la glacière quand les enfants ont demandé...

If you said les enfants demandaient, it would suggest a more ongoing or repeated action, which is not the main idea here.

In standard French indirect questions, the usual word order is:

• question word + verb + subject

So:

• où étaient les serviettes = where the towels were

This is an indirect question after ont demandé:

• les enfants ont demandé où étaient les serviettes

Compare:

Direct question:

• Où étaient les serviettes ?

Indirect question:

• Ils ont demandé où étaient les serviettes.

French keeps the same order here.

Because the subject is les serviettes, which is plural.

• la serviette = the towel
• les serviettes = the towels

So the verb must agree with the plural subject:

• où était la serviette = where the towel was
• où étaient les serviettes = where the towels were

That is why French uses étaient, the plural imperfect form of être.

It can mean either, depending on context.

• serviette can mean towel
• serviette can also mean napkin

In this sentence, the meaning shown to the learner probably tells you which one is intended. Without context, French speakers may rely on the situation:

• picnic / beach / cooler context → towels is possible
• meal / table context → napkins is possible

If someone wants to be more specific, they might say:

• serviette de bain = bath towel
• serviette de plage = beach towel
• serviette de table = table napkin

Because French often goes straight from asked to the indirect question.

English can say:

• The children asked where the towels were

French does the same:

• Les enfants ont demandé où étaient les serviettes

You do not need que here, because this is not a regular that-clause. It is an indirect question introduced by où.

Compare:

• Ils ont dit que... = They said that...
• Ils ont demandé où... = They asked where...

Yes, and this is a good point to notice: French indirect questions work very naturally here.

After verbs like:

• demander = to ask
• savoir = to know
• dire = to say/tell, in some contexts

French can introduce an indirect question with words like:

• où = where
• quand = when
• pourquoi = why
• comment = how
• ce que = what

So:

• Ils ont demandé où étaient les serviettes is perfectly normal French structure.

A useful way to read it is in chunks:

• Tu étais en train de remplir la glacière

  • tu étais = you were
  • en train de remplir = in the middle of filling
  • la glacière = the cooler

• quand les enfants ont demandé

  • quand = when
  • les enfants ont demandé = the children asked

• où étaient les serviettes

  • où = where
  • étaient = were
  • les serviettes = the towels

So the sentence has:

1. an ongoing background action
2. a completed event
3. an indirect question reporting what they asked

That is the basic structure of the whole sentence.