collider is changing if it's rotating and if it's a child of something

when I parent a rigidbody (with a box collider) to something, and make it rotate, the collider bounds change.

here's an example of how it get's stretched (or thined) when a ragdoll lifts his leg

should I never "en-children" objects with colliders? >.>

or is there a work-around?

What you seem to have here is a “scale” problem.
Basically, to avoid many difficulties, your armature (the transforms hierarchy of the gameobject) should have a scale of 1, and the same goes for the mesh.

Additionally, you might want to check that the anchor of the rigidbody is in the same spot of the transform’s PIVOT, and that the collider are correctly centered in respect to their transform!