Milestone Federated Architecture (MFA) is only supported using AD/Windows users. If you use the Config API Client sample and an AD user you can see the Child Sites is being populated.
"The SiteInfo.xml request is today only useful for AD users since basic users are considered site private. This request is planned to be discontinued in the next release, 2020 R3.
The same information can be retrieved using the Configuration API. This will also work with future authorization methods, including basic users. The top path for getting this information is: ‘/SiteFolder’."