Can the root user of an organization top-level account access resources in child accounts?

The root user of an AWS account can't directly access its child accounts. AWS Organizations allows managing multiple accounts but it doesn't give root users automatic access to child accounts.

To access a child account from the top-level one in an emergency, you would have had to set up cross-account access in advance. This typically involves creating a special role in the child accounts that the top-level account can assume.

Below is some related documentation that should help.

AWS Organizations: This is the general overview page for AWS Organizations which talks about managing multiple accounts: https://aws.amazon.com/organizations/

Creating a Role to Delegate Permissions to an IAM User: This guide walks you through creating a role for cross-account access: https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_create_for-user.html

Tutorial: Delegate Access Across AWS Accounts Using IAM Roles: This tutorial is specifically about delegating access across multiple AWS accounts: https://docs.aws.amazon.com/IAM/latest/UserGuide/tutorial_cross-account-with-roles.html

I hope this was helpful, if so please remember to mark this answer as accepted. Good luck!