Example for bevy_rapier to illustrate an issue that I have but haven't yet figured out how to solve

children_colliders.rs

use bevy::prelude::*;
use bevy_rapier2d::prelude::*;
// use bevy_inspector_egui::quick::WorldInspectorPlugin;

fn main() {
    App::new()
        .insert_resource(ClearColor(Color::rgb(
            0xF9 as f32 / 255.0,
            0xF9 as f32 / 255.0,
            0xFF as f32 / 255.0,
        )))
        .add_plugins((
            DefaultPlugins,
            // using the inspector to discern what are the entities detected by the collision event
            // WorldInspectorPlugin::new(),
            RapierPhysicsPlugin::<NoUserData>::pixels_per_meter(100.0),
            RapierDebugRenderPlugin::default(),
        ))
        .add_systems(Startup, (setup_graphics, setup_physics))
        .add_systems(PostUpdate, display_events)
        .run();
}

fn setup_graphics(mut commands: Commands) {
    commands.spawn(Camera2dBundle::default());
}

pub fn setup_physics(mut commands: Commands) {
    /*
     * Ground
     */
    let ground_size = 500.0;
    let ground_height = 1.0;

    commands.spawn((
        TransformBundle::from(Transform::from_xyz(0.0, -ground_height, 0.0)),
        Collider::cuboid(ground_size, ground_height),
    ));

    /*
     * Create a Cube with the shape of a |_|
     */
    commands
        .spawn((
            TransformBundle::from(Transform::from_xyz(0.0, 100.0, 0.0)),
            RigidBody::Dynamic,
        ))
        .with_children(|children| {
            // Left Piece
            children.spawn((
                TransformBundle::from(Transform::from_xyz(-20.0, 0.0, 0.0)),
                Collider::cuboid(2.0, 20.0),
            ));
            // Right Piece
            children.spawn((
                TransformBundle::from(Transform::from_xyz(20.0, 0.0, 0.0)),
                Collider::cuboid(2.0, 20.0),
            ));
            // Bottom Piece
            // since it's at the bottom, it's the only one colliding with the ground
            children.spawn((
                TransformBundle::from(Transform::from_xyz(0.0, -20.0, 0.0)),
                Collider::cuboid(20.0, 2.0),
                // We can enable collision events here or at the Ground collider so they are detected
                ActiveEvents::COLLISION_EVENTS,
            ));
            // little cube at the center, just for reference
            children.spawn((TransformBundle::IDENTITY, Collider::cuboid(2.0, 2.0)));
        });
}

// We detect a collision here between Bottom Piece and Ground
// due to the cube having a Rididbody::Dynamic causing gravity
// and to the ActiveEvents::COLLISION_EVENTS on the Bottom Piece
fn display_events(mut collision_events: EventReader<CollisionEvent>) {
    for collision_event in collision_events.iter() {
        println!("Received collision event: {collision_event:?}");
    }
}

// ... However, I want to destroy the whole cube entity when touching the ground...
// but I only detect Bottom Piece and Ground entities... :(

// How can I detect any collisions that the children of a Rigidbody have?
// So I can detect here a pair of Cube and Ground

// Sure, I can use the Parent component and despawn its Entity
// but, what if the children colliders are in a deeper descendant?