Answer:
Sentémonos.
Step-by-step explanation:
The phrase "let's sit down" is a command and It can be translated in two different forms:
-The first one could be: Sentémonos.
-The second one could be: Vamos a sentarnos.
In Spanish pronouns usually are attached to the verb as in the first example. This doesn't happen in English because pronouns are separated from verbs.