In English it's easiest to find adverbs when they are used immediately before the verb. However, Duo recognized "Let's quickly hide in the park!" as a typo, even though the "ly" word was used in a typically correct order in English. So, it should be noted as another correct solution without the "typo" indication.
To hide quickly...most often after a verb.
These are different combinatiins. Each one guven more imoortance on a different part of the statement.
Hide quickly! In the park!
In the park! Hide quickly.
Quickly, hide in the park!
..... Punctuation makes a difference. We'll need outside sources if Duolingo languages do not teach it. I can be very important.