Here's my .02, as a DVC owner at BLT, a mom, and someone who's also trying to make the same decision on reception spaces for our VR!
The view from Napa is nice, but I think it's better at night. During the day, you'll see lots of concrete and parking lot between you and the castle. If that doesn't bother you, then no problem.

For myself, I prefer looking at the MK at night ... it's easier to overlook everything in between when it's dark.
With Whitehall, you've got a lot of space outside of the room if you include the covered porch area and the patio. With little ones that will get restless just sitting for a long time, you might find that patio area to be a huge blessing, giving them somewhere to run around a bit and let off steam.
Also, are you having a Wishes or an Escape? With Wishes, you don't have to meet a specific venue minimum at Whitehall -- just the minimum DFTW sets out for the time of day. With Napa, you have to hit DFTW's minimum and the venue minimum (which doesn't include the cake because you're not at the GF). That might make a difference, too.
Oh -- and I don't think you can alter the menus offered at Napa/California Grill at all. At Whitehall, you can serve whatever you'd like.
HTH a bit! Let us know what you decide.