Hello
I have a hand i sliced off the main character mesh from blender.
in Unity I add this hand as a child of the original hand.
When I test the game there is just a single idle animation but the added hand does not follow the animation, it just stays in place.
What is the approach here?
I am trying make a FPS where I want to shoot off the hand
This is just a guess, since you don’t give any details at all: if your character uses skeletal animation, simply parenting a mesh won’t make animation work: you need your mesh vertices to be weighted to the bones, which are the thing actually driven by the animation.
and by the way, this question isn’t related to physics in any way so wrong forum! 
good luck!