Update breakout example's stepping plugin to use children (#18271)

# Objective

Contributes to #18238 
Updates the `SteppingPlugin` of the `breakout` example to use the
`children!` macro. Note that in order to test this usage you must use
`--features bevy_debug_stepping` and hit the back-tick key to enable
stepping mode to see the proper text spans rendered.

## Solution

Updates examples to use the Improved Spawning API merged in
https://github.com/bevyengine/bevy/pull/17521

## Testing

- Did you test these changes? If so, how?
- Opened the examples before and after and verified the same behavior
was observed. I did this on Ubuntu 24.04.2 LTS using `--features
wayland`.
- Are there any parts that need more testing?
- Other OS's and features can't hurt, but this is such a small change it
shouldn't be a problem.
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
  - Run the examples yourself with and without these changes.
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
  - see above

---

## Showcase

n/a

## Migration Guide

n/a

examples/games/stepping.rs

@@ -161,8 +161,7 @@ fn build_ui(
         stepping.always_run_node(label, node);
     }
 
-    commands
+    commands.spawn((
-        .spawn((
         Text::default(),
         SteppingUi,
         Node {
@@ -174,12 +173,8 @@ fn build_ui(
         },
         BackgroundColor(Color::srgba(1.0, 1.0, 1.0, 0.33)),
         Visibility::Hidden,
-        ))
+        Children::spawn(text_spans),
-        .with_children(|p| {
+    ));
-            for span in text_spans {
-                p.spawn(span);
-            }
-        });
 }
 
 fn build_stepping_hint(mut commands: Commands) {