The short answer
Usually from about six. Most riad-based classes welcome children aged six and above, but check each school's policy when you book. Children tend to enjoy the souk walk and the bread-making most. Many classes use gas or induction hobs for the main cooking, and very hot charcoal sessions are best avoided with young children.
In more detail
A cooking class works well as a family half-day because it mixes walking, touching and eating. Many begin with a walk through the souk to choose vegetables, herbs and spices, which children enjoy: smelling cumin, watching saffron weighed, learning that the lanes are named after their trades.
In the kitchen, bread is the hit. Kneading and shaping khobz, the round daily bread, or folding msemen gives small hands a job. Tagine and couscous classes follow, and everyone eats what they made at the end with mint tea.
Ask a few questions before you book: the minimum age, how hands-on children can be, whether cooking is over charcoal or a gas or induction hob, and the group size. A small private class is easier with children than a large group session. Mention any allergies or diets when you book and again the day before.
Places in Marrakech that teach classes include La Maison Arabe, the Amal women's training centre in Targa and Café Clock in the Kasbah. We can arrange a class as part of your trip; tell us the children's ages.