Update render_primitives example for children! macro

[krunchington]

Objective

Contributes to #18238
Updates the render_primitives example 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

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Migration Guide

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