My thoughts are do we break the trip between Yosemite and Death Valley, or do we add a night in Bryce, Zion or Monument Valley?
My immediate thought when looking at your itinerary were to be honest, visit Zion and Monument valley in a loop to the GC from Vegas and break down the 15 nights you have around LA and SD. You could spend just the 1 night in Page and one night in the lovely town of Springdale UT for Zion, but adding another night or 2 would be beneficial imo. That night in Vegas you are unsure of could be added to, Bryce canyon and MV are pretty special too.

Also I have read on various other sites that Sep is the worst time to visit Yosemite.
I don't understand this at all, it's a wonderful time to visit Yosemite !! The only downside is that the waterfalls are dried out at this time of year, but they are usually in July and August and beyond Sept until the Spring snowmelt again. The thing is the temps are usually good and you don't have the heat and crowds of the summer. Do not skip Yosemite, it is an amazing place !

I'm sure the Kids will love the theme parks, but don't underestimate how much fun Kids of that age will have among nature. Running around, spotting wildlife and playing by the river are the things they tend to remember as they get older. You can also sign then up to Junior Ranger programs in the National Parks where they collect badge and learn interesting facts.

The other thing you could consider is doing your trip in reverse order. The main benefit would be having the Ocean on your side of the road heading south to LA.

You are in for a great trip !