Ah, my apologies - I thought you were talking about the context of parent and child tasks, having forgotten that you can have 'parent contexts' which include one or more 'child contexts'. I'd even forgotten that the option to include contexts within other contexts exists.
Unfortunately, as soon as something is assigned the context "morning", it will be automatically considered to be included in the context "time of day", because you've defined "time of day" to include "morning". Contexts can be included within other contexts, but it's not the same as the hierarchy of parent and child tasks.
This possibly makes your context assignment easier, though: You don't have to assign "time of day" directly to anything - all of the items with "morning", "afternoon" or "evening" would be visible in any filtered view using the context "time of day".