The short answer
Plenty. Marrakchi food is gentle on young palates: chicken tagine with olives and lemon, couscous, harira soup, grilled brochettes, msemen pancakes with honey, fresh bread, oranges and the square's fresh juice. Most riads will cook plain grilled chicken, rice or eggs for fussier eaters and tone down the seasoning. Stick to bottled water and cooked food.
In more detail
Moroccan cooking is spiced rather than hot, so most dishes suit children. Breakfast is the easy win: msemen, a flaky folded flatbread, and baghrir, a pancake full of holes, both with honey. At lunch and dinner, mild tagines, couscous and brochettes are on every menu. Mint tea is sweet enough for most young palates.
Treats are easy to find. The orange-juice carts of Jemaa el-Fnaa squeeze to order. Gueliz cafés serve crêpes and pastries. Sweet & Sook, a small ice-cream counter in the Mouassine souks, is an easy stop when you are exploring with children, and Panna Gelato near Bab Agnaou works well after the Saadian Tombs or El Badi Palace.
A few habits keep stomachs happy. Use bottled water for drinking. Prefer cooked food to raw salads rinsed in tap water, and skip unpasteurised dairy from market stalls. The younger the child, the more this matters.
Riads and visitor restaurants will happily make plain food at off hours, and many have high chairs. Pack familiar snacks for the long driving days into the Atlas and the desert, where the choice is thin.