I have wanted the same thing. There is no elegant way of solving this today. You can take those tasks from job A and extract into a separate job B. In job B you click on permissions, and remove ability to execute. Job B can no longer be executed manually. In job A you create a flow where you have the condition On success => run job B.
I think this should work, but it's a bit messy, and overriding permissions to achieve this is not ideal. It would be nice to be able to have a checkbox in each task, where you have the ability to define it as a 'Child task only'. This would imply it cannot be executed manually.