Learn Russian: Дети мылись, а потом завтракали. - The children were washing themselves, and then they had breakfast.

Reflexive verbs have two endings: -ся and -сь. You use -ся after consonants (like in the infinitive мыться) and -сь after vowels. Since the past tense plural мыли ends in the vowel и, we attach -сь to make мылись.

Does мылись mean they completely finished washing?

Not necessarily. Мылись is an imperfective verb, which focuses on the process or action itself rather than the completion. It means they were in the process of washing themselves. The same goes for завтракали (were eating breakfast).

Why use а here for 'and' instead of и?

While и means 'and' for simply adding things together, а is often used to show a transition to a new action or a sequence of events. The phrase а потом is the standard way to say 'and then' in Russian.

Is дети a regular plural word?

No, it is a very common exception you have to memorize. The singular word for 'child' is ребёнок, but the plural 'children' is completely different: дети. Since дети is plural, both verbs in this sentence use the plural past tense form (мыли, завтракали).