move all_children functions to GA

[jchen293]

Description

• Adds all_children function to the User class for retrieving paginated lists of children users, and deprecate the beta function
• Adds get_next_page_of_children function to the User class for retrieving next paginated lists of children users, and deprecate the beta function

Testing

Add unit tests for GA functions

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)