14-15 days is really tight for Madrid, Barcelona, Seville, and Fes/Marrakech, and that's omitting the beach. Morocco is fabulous, you'll be comfortable traveling as a family (for a woman alone it's not unsafe, but...different), and it will be quite an experience. But you have so little time. You will be giving up some wonderful sights/cities in Spain to spend time in Morocco, but every trip is a matter of making tough decisions.
Madrid's average December high temperature is 50F; Barcelona's is about 9 degrees higher, so odds are that the weather will be more pleasant in Barcelona.
I always like some architectural variety on my vacations, so the one thing I would try to avoid here is a trip spent entirely in Andalucía and Morocco. Although Cordoba, Granada, Ronda, etc. are lovely and pair well with Seville, geographically speaking, it would not be my preference to try to squeeze in all of them in lieu of Madrid or Barcelona. So I'd try for Madrid or Barcelona (probably not time for both), Seville, and Fes/Marrakech. You can research side-trips from each of the Spanish cities and leave the final decision until you arrive and see how much time you want for sightseeing in the base cities. Be aware that you will be jet-lagged (and at least some of you probably zombie-like) on your arrival day, so you probably won't be very productive then.
You should consider flying into or home from Morocco (that would likely be Casablanca, unfortunately). That way, you'd just have a one-way flight to Spain as you continued your trip.
There are great shopping opportunities in Morocco for crafts. Fes seems to be the craft capital, but the market in Marrakech is also excellent. That might argue for ending your trip in Morocco so you don't have to cart ceramics, copper, woodwork, carpets, etc., through Spain.