A place for Software Engineering...
Interesting thought about the hierarchy. I consider software engineering to be a 'doing' thing with defined practices and principles. Whereas a 'science' is the academic theory and discovery of new things. Would you say that Automotive Engineering at a motor manufacturer is a 'science' or more 'engineering'?
The BCS Glossary of Computing Terms tenth edition states 'Software Engineering' as "is the science of designing and constructing new or modified computer systems, based mainly on computer software (programs)". But have things now changed from science to engineering? And that the practice of creating software is an engineering process, but the development of new software engineering techniques is a 'science'.
I think that perhaps the division into Computer Science, IT and Digital literacy was a pragmatic one about not wanting too many areas.
Looking at the content of the Draft PoS for Computing it looks like the doing bits (that you probably quite correctly label Software Engineering) are subsumed within Computer Science.
This is also what happens when you get to GCSEs - I'm not aware of there being a Software Engineering GCSE, but Computer Science GCSEs contain lots of programming.