We are planning a trip in late May to early June for Spain and Morocco with our children who will be 18m, 17f, 15f, and 11m at the time of travel. We plan to open jaw either flying into or out of Casablanca or Madrid to home in San Francisco. For the Spain portion of the trip, Barcelona, Madrid and Seville are non-negotiable. The middle girl wants to see Gibraltar. For Morocco, the kids really want to experience the souks and the desert. Somewhere in there, my husband will want a day on the beach. How would you divide up the days and how difficult is customs between Spain and Morocco? Is there a direction that is easier?
Honestly, 15 days is barely time to cover your non-negotiable points in Spain, let alone have any room to make Morocco with the effort.
Here’s another vote for just staying in Spain. You could add Granada and do 3 or 4 nights in each place. There’s plenty to do in each city and they’re all different from one another.
Casablanca is a modern city. It's probably the only available gateway from the US, but Fes and Marrakech are vastly more interesting from the tourism point of view, and they have flights from many cities in Europe. You don't have to make your trip to Morocco this year just because you'll be in Spain.
I agree that you don't have enough time for what you want to see--not even close.
There are flights from some Spanish cities to Fez and Marrakesh, the two best places to visit. You'd need 2 days just for the desert experience - it's not "just down the road" from either and you'll want to be there for either sunset or sunrise, or even both.
Morocco is a wonderful tourist destination. Plan a separate trip there for 2 weeks. You won't regret it. 15 days - even if that doesn't include the days to/from SFO - is just enough for Barcelona, Madrid and a taste of Andalucia. If possible, fly open-jaw Barcelona/Madrid.