THESE DAYS vs SINCE

On last Friday the kids went to live with their grandma for a few days. People ask about them and of course I say a continuity "The kids live with their grandma". However, I want to give it a timing and find it very interesting.

If we add 'these days', it is still a continuity:
Ex: The kids live with their grandma these days.

But if we add 'since last Friday', the Simple Present cannot be a continuity:
Ex: *The kids live with their grandma since last Friday.

To keep the impression of continuity, we have to change the tense from Simple Present to Present Perfect:
Ex: The kids have lived with their grandma since last Friday.

How shall we explain this phenomenon? Have you ever noticed this? Time adverbials seem to be different, don't they? Your opinion is welcome.

Shun Tang