Where do I find padlock key in granny chapter 2 ?

The padlock key can be found sometimes inside slendrina's husband coffin to get their you need first the following: Security Key(To access the security room) Stun gun with ammo or Shotgun with ammo (To shoot the red target to open the cage) Cutting pliers (To cut the metal chain) And you can find it but sometimes the padlock key isn't there.

Sometimes it is in the piano

The key and the other items all spawn in different places each time you start the game. You will need to check cupboards and boxes (with a crowbar) around the house to find it.

Check the house guide for some possible locations.