refactor: support get_child_blocks along with get_child_descriptors

[Agrendalath]

Description: This method will be renamed in the Redwood release.

Dependencies: This is required to support changes from openedx/openedx-platform#31492.

Testing instructions: See that the tests from openedx/openedx-platform#31492 are passing.

Reviewers:

• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings): n/a
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPi after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[openedx-webhooks]

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[openedx-webhooks]

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[Agrendalath]

@ormsbee, would you mind creating a release to push the new version to PyPI?

[ormsbee]

@Agrendalath: Done (thanks to @kdmccormick, who clued me in to how we're using GitHub release now).