Fantasia is great family entertainment and the seaside at Essaouira is an excellent excursion for children. Imlil is a good choice if your children are comfortable walking on uneven terrain and Ourika is a fun option if they would enjoy an hour and a half or so walking and scrambling up the river bed to make it up to the waterfalls.
The Kasbahs, the Grand Tour and Waterfalls and Monkeys all involve a lot of driving and are less suitable. We would definitely not recommend the night in the desert which is a really ambitious trip that even some adults find tough going.
Car seats can be provided when they are clearly requested at the time of booking your excursion, and when your booking is made at least 48 hours before departure.