Where can I find details/instructions for this keychain?
While searching on Bricklink I've found two items that are related to minifigure in question. All I could find is some picture and minor details of this minifigure flashlight key chain here.
As we can see on picture below one of two of the items I've located on Bricklink are present in this bundle. For some reason items in Bricklink's catalog share similar part ID (UT21222-1 and UT21222-2, where &quot;-number&quot; is used to distinguish items with same part ID), while KAI has entirely different picture. It is unclear if picture used on Bricklink is incorrect or Kai's print has changed at some point.


Some details from shared website (in German):

Art.Nr.   3017.21222
Hersteller Nr.    UT21222
EAN-Code  4041268212220


Schlüsselanhänger mit Taschenlampe im Lego Ninjago Design inkl. Batterien im Display Hiermit stehen Sie nicht länger im Dunkeln. Durch die beweglichen Beine kann gleichzeitig in zwei Richtungen geleuchtet werden. Im Lieferumfang sind bereits zwei Knopfzellen (Typ CR2025 3V) enthalten, welche bei Bedarf ausgewechselt werden können. ACHTUNG! Nicht geeignet für Kinder unter 36 Monaten. Enthält verschluckbare Kleinteile. Erstickungsgefahr!

And with a help of Google Translate in English:

Keyring with torch in Lego Ninjago design incl. Batteries in the display With this you are no longer in the dark. The movable legs can be used to illuminate in two directions at the same time. The scope of delivery already includes two button cells (type CR2025 3V), which can be exchanged if necessary. ATTENTION! Not suitable for children under 36 months. Contains small parts that could be swallowed. Suffocation hazard!

Unfortunately, I think LEGO does not keep/provide any online instructions for this accessory.