"Let's play instruments."
Translation:E hoʻokani i nā pila.
The problem is that the English doesn't match the Hawaiian. "E hoʻokani i nā pila" can only be translated as "(You) Play the instruments." If you want to say "Let's play instruments," either "kāua" or kākou" cannot be deleted. They're both considered imperative sentences, but it's the difference between a direct command to "you" (which can be deleted) and a suggestion saying "let us," and the "us" in a suggestion can never be deleted or it will be misunderstood as a direct command to "you." Hope this helps.