Why preterite, not imperfect?

Hola Shirley

The combination of the preterite and the imperfect is also common  in sentences like these. Both options would be valid: 

1. Los niños jugaron en el parque mientras los adultos nos tomábamos un café y charlábamos.

2. Los niños jugaban en el parque mientras los adultos nos tomábamos un café y charlábamos.

In 1 and 2 the speaker is seeing the action about the adults having coffee and chatting as something more descriptive, what was happening, ongoing action... but in 1. the speaker sees the playing action as something that had a beginning and an end, it's seen as completed (so, for as long as the parents chatted and had their coffee they played, and then there was an end to it). In 2, however, the speaker is expressing the action of playing as more descriptive, something that was happening during that time of chatting and having coffee, but the speaker doesn't want to convey that the playing had an end. 

Sometimes, to understand the use of one tense or another, we need to think what the speaker wants to highlight/convey when saying it. There is no clear indicator that would make you think you need one tense or another, it is a question of trying to see what the speaker at that moment is thinking of that action. 

To give you a simple example, you can say for example:

Ayer llovió mucho, or 

Ayer llovía mucho.

The fact that we are using "ayer" as a time frame doesn't always mean that we cannot use the imperfect (although as a beginner you will learn that ayer triggers the preterite). If I say ayer llovía mucho, using the imperfect, I, as a speaker, am seeing the action of raining as something happening continuously during the day of yesterday, describing what was happening. This usage is less common because this nuance if specific and it may happen less that the usual ayer llovió, but it is a possibility. 

Using the preterite vs the imperfect

I hope it helped.

Saludos

Inma