How do I animate a 2D weapon?

I don't get all this animator layer stuff, as I am a beginner and could really use some help in the right direction. I'm not even sure if that's what I'm gonna have to use. So I'm watching this game tutorial for unity where they're making a topdown 2D rpg and they add a weapon to the character as a "child". The player has a "child" called weapon and the weapon has a "child" which is the actual sword. This is to make sure that when I switch weapons it will end up in the right position, as I've understood. Now this person who makes the tutorials doesn't make the sword move, but I've already drawn a sword animation. However, when I click "add property" to the animation of the player where he swings his arm and then click "sprite renderer", I can't select the sprite to animate it. The sprite is stiff and can't be animated. The player arm moves but the sword is stuck in place. I can only change it's position and rotation. What am I supposed to do??