Help the exhausted parents finish their text message to the babysitter.

We finally sat down to enjoy a warm cup of coffee _____ the energetic toddlers had completely destroyed the living room and fallen asleep.

The correct answer is after.

We use after to show that one event happened later than another. Here, the parents sat down later than the toddlers falling asleep.

The past perfect tense ("had completely destroyed and fallen asleep") strongly pairs with after to emphasize that the chaotic toddler tornado was fully finished before the parents got to relax.