One problem with block scheduling is that "distributed practice" is more effective than "massed practice." Your brain needs to encounter something, let it go, encounter it again, let it go again, and so on. So doing a subject several times a week will be more effective than doing it just once a week.
Of course, you need to balance that against the transitional problems you mention, and also the need for large chunks of time to get deeply involved in more complex projects, as a kid gets older. But the distributed practice issue is one factor to consider.
