Update help text copy in the child items widget

What does this MR do and why?

• Update help text within the Child items widget
• Resolve the first iteration of "Child widget empty state 🎨"

Screenshots or screen recordings

Change help text copy from: No child items are currently assigned. Use child items to prioritize tasks that your team should complete in order to accomplish your goals! to No child items are currently assigned. Use child items to break down the goal of this issue into smaller parts.

Before	After

How to set up and validate locally

• Create an issue
• View the Child widget under the description area of an issue

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363457 (closed)