Collider2d cast to player child ignores the child

I'm trying to implement robust collision detection for a platformer. Collider2d.Cast() seems like the perfect thing to use for that. However, the platforms are slimmer than the character sprite, and sometimes a side collision cast upwards still sees player sprite, therefore registering a downward collision when it's not one.

I tried applying a rigidbody child "leg hitbox" to the character sprite and setting it to a new layer, then masking that layer, however I must be missing something about how filtering or casting to children works because cast seems to ignore the child.

Here's the relevant snippet. What am I missing?

 ...
 RaycastHit2D[] hits = new RaycastHit2D[10];
 ContactFilter2D filter = new ContactFilter2D();
 filter.SetLayerMask(LayerMask.NameToLayer("Player Collision"));
 filter.useLayerMask = true;
 if (collision.collider.Cast(Vector2.up, filter, hits) > 0)
 { ...