Computing Curriculum

I have been looking to begin my preparation for the new computing curriculum requirements and have come across a most interesting question.

Should the development of a Computing scheme be 'top down' or 'bottom up'?

I have been looking at the Computing for Schools Curriculum  - https://drive.google.com/file/d/0B4s7hMWhfvHlTnZtZ0FmRVczeEk/edit?usp=sharing
For those that have not accessed it.

Whilst we know what we wish to reach at the end of KS4, it is surely a self defeating process to continually work backwards from here?

Over the past 4 years, I have found that I have been shuffling certain elements of programming down through the year groups. Year 4 are perfectly able to understand and use Scratch independently, with this comes an understanding of the processes required to provide a computer with instructions. This begins to sound much like the requirements for the end of KS2/KS3.

When I set out to plan my computing element for the new year (this is not to say that we do not already teach much of the concepts) I am going to look at what my youngest CAN do, what they NEED to do and what they COULD do. My annual review of computing is going to have to stem from this, taking work that we can from higher age groups and then creating new projects higher up the school. Otherwise I fear that a hole will be created where harder concepts are achieved by the top, without necessarily ensuring that the work at the lower level progresses to meet these levels.

With much to do, I want to continue to look for free software that the pupils can download at home to extend knowledge, but my head is continually turned by physical objects such as Lego mindstorms that could bring a whole new level to our programming.