As it is documented, using the Microsoft Graph REST API you can (among other options) get an item by Id or Path. This works fine, as expected:
var items = await graphClient.Me.Drive.Items[myFolderId].Children.Request().GetAsync();
This is how:
var items = await graphClient.Me.Drive.Root .ItemWithPath("/this/is/the/path").Children.Request().GetAsync();
Use just the plain path. Don't include the ":", and don't include the "/drive/root:/".
it was obvious, now that I see it...