…se children macro (#18318)

# Objective

Contributes to #18238 
Updates the `sprite_slice`, `spatial_audio_3d` and `spatial_audio_2d`
examples to use the `children!` macro.

## Solution

Updates examples to use the Improved Spawning API merged in
#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

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## Showcase

n/a

## Migration Guide

n/a