Well during tha winter months California and Texas are two of the warmest areas if you don't want to travel outside of the United States
I would say its Southern Greece such as Crete and Rhodes.
The temperature in these places in the following months:-
18°C in November,
16°C in Dec., and
14°C in Jan.
Costa Rica! I would suggest South America, the Caribbean, or Australia.
Assuming you live in the US or above the equator for this matter, travel anywhere below the equator for the seasons are reversed. If you need some ideas, check out best family winter vacations and see if any of these places fits.
EGYPT is a lovely place in the winter to visit, never rains always sunshine. Great people, nice hotels, staff waiting on you all the time, especially if you go 4 or 5 star.
Goa in India is a fab destination for winter sun and the food is amazing as well!!