I think it's all relative to time. Is a half day at Niagara better than nothing? Absolutely. Ditto Yosemite. We're just giving our opinion relative to the other options. The more we post why we did or did not like a spot, the more informed the OP is to decide what is best for his family.
But there is general agreement this is very aggressive schedule and I'm more aggressive than most on grand trips. The back end needs some relief built in so something has to get cut. And the time/speed/distance simply doesn't work for SD to Yosemite. There is 100 miles of city driving on that route that becomes a parking lot every day if you don't plan appropriately. And if you do plan appropriately, leaving SD around 9:00 AM, you will still have a 9-10 hour drive ahead of you to get to Yosemite without factoring stops. You're in CA so I'm sure you know what I'm talking about.
The alternative, is hit all those spots and add two weeks - but he has a festival at the end which will drive the return date. I don't think the question is whether a half day is enough but rather a half day better than nothing?