In this sentence we would use Präteritum. In general you use this tense, to describe a situation (like in the example), or also to describe how something was at a certain time, like the background of a story: Das Wetter war schön. Wir ließen es uns gut gehen..... You are right, that the Präteritum is usually only used for "written German" and that many natives use Perfekt when talking to each other. That's common practice, even though it's grammatically wrong at times. "Lassen", however, being a verb of "description" is commonly used in the past tense (as you can see in the example I gave). Further examples: Wir ließen uns die Sonne auf den Pelz scheinen. Wir ließen die Seele baumeln. Er ließ das Essen anbrennen. ABER: Er hat sich nichts zu Schulden kommen lassen. Here we have the connection to the future, so we would use Perfekt, meaning that up to now, he has not done anything wrong.

Fuer den muendlichen Ausdruck auf jeden Fall: Die Eltern haben die Kinder nicht ins Kino gehen lassen.

I would definitely use Perfekt: "Die Eltern haben die Kinder nicht ins Kino gehen lassen".

Oh ok. Thanks DonHolgo. I didn't know that before.

The answer would depend on the context. Die Eltern haben die Kinder nicht ins Kino gehen lassen. --> Dieser Satz ist sicher nicht falsch.