In reply to TobyA:
I've been to Morocco several times, including twice with my daughter; first when she was about 5, trekking in the Atlas, then when she was about 11, trekking in the Sahara with camels. October was when we took her both times as it is so much cooler and therefore more pleasant for walking. She loved it and I'm pleased to say still loves trekking and climbing with us, even though she is now of an age to start rebelling and wanting to do her own thing.
Usual rules apply about being sensible and sensitive when meeting the locals, especially in the souks in Marrakech. Don't carry too much cash and don't flash it around. The Moroccans are family orientated so will be pleased to see a British family visiting, and will probably try to spoil you kid(s) rotten. In all my visits I've never had any problems in Marrakech, the mountains, the desert, or at the coast.
I've not tried to drive there and really wouldn't want to, travel is pretty cheap and efficient so there is no need to drive oneself.
Having time out in Essaouria is definitely a good thing, there is a far more relaxed vibe to the place, far easier to browse the handicraft shops etc, without being dragged bodily in and if you say ´No' then that's it, they leave you in peace.
Have a great time.