To use Insider Risk Management, including Information Barriers, all users who will be affected by the policies must have a license that includes this feature. This means that all users who need to be prevented from collaborating and sharing information with other child companies must have the appropriate license.
In this scenario, if only certain child companies need to be prevented from collaborating with others, then only the users in those child companies need to have the appropriate license. However, if all users in the tenant need to be subject to the information barriers policies, then all users will need to have the appropriate license. To determine which license is required for Insider Risk Management, please review the Microsoft licensing documentation. Here are some links to the Microsoft documentation for more information on Insider Risk Management and Information Barriers: