The short answer
It is a big day: about 3 hours 45 minutes each way to Aït Ben Haddou over the Tizi n'Tichka pass, rated moderate. Older children often love the earthen kasbahs and the Atlas Film Studios in Ouarzazate, but for young children it is a lot of car time. Many families do it as an overnight instead, or save it for a longer trip.
In more detail
The road is the challenge. It climbs the High Atlas over the Tizi n'Tichka pass at 2,260 m, with hairpins, photo stops and a Berber tea house at the summit, before dropping into the pre-Saharan south. Counting both directions, that is around seven and a half hours in the car, before the Ouarzazate leg.
The reward is real for children who can manage it. At Aït Ben Haddou you cross the river and climb the earthen village with a local guide, a scramble that older children enjoy. In Ouarzazate, the Atlas Film Studios, an optional visit, are a surprise favourite with children.
For younger children, split it. On a family loop, a night in a kasbah guesthouse at Aït Ben Haddou turns two long drives into two half-days, and it fits naturally on the way to the Sahara. Pack layers, since the pass is cold and the south is hot, plus snacks and anything for car sickness on the hairpins.
If this year is not the year, the Ourika Valley or Agafay give a taste of the mountains or the desert with far less driving.