I would choose Newport Coast Villas. We go either in December or January every year on a getaway and I swim every day. The pool was like bath water
LOL! What you think of as a bathwater temperature pool is freezing water to locals. We wear *parkas* down here when it gets below 60F.
I've been to NCV in Feb - similar weather usu to Dec - and the pool was cold but kids will still go in it. If they aren't from a hearty Northern stock or don't constantly move, even kids will get cold and want the hot tub to warm up. You can get lucky and get some really nice weather in Dec, but if you are talking this Dec, El Nino is supposed to be rearing its head so I wouldn't bank on lots of 70-75F+ weather. On the other hand, occasionally we get some exciting waves with the winter storms and that can be fun to watch.
The views at NCV are really nice so you might miss the view if you go to a desert resort, but the pools at Shadow Ridge are *great* for kids. There are also plenty of activities in the Palm Springs area - The Living Desert, Tram, Joshua Tree daytrip, etc.