I went to CA Christmas 2011 & swam in outdoor pools. It was lovely & I am a fair weather girl, so not that hardy! LA at lunchtime was warm enough for a dip but we wore t shirts & jeans around town and definitely needed a jacket in the morning & evening. Palm desert was lovely around lunchtime too but jackets needed at night. Go for it. (& just think the queues at universal studios & Disney will be much smaller than in the summer too)
We went to California for a few days pre cruise during Feb half term a few years ago with children a similar age to yours. The hotel pool wasn't usable as to cold. There was plenty of other things to do but if swimming is your thing I wouldn't go in the winter. To explain the temperature , the first day I wore three quarter length trousers and a fleece, one day jeans and a much needed rainmac and one day three quarter lengths and a t shirt.
We are thinking of going to California for Christmas and was wondering if it would be too chilly to enjoy it properly? I know it won't be freezing but often pools aren't heated and we do love swimming, so would this put you off? Going with 2 kids aged 7 and 10. Would we be better off going in the summer instead?