The short answer
Yes, it is one of the better ones. The day trip is rated easy, the town is walkable with a carrier, and there is a windswept beach, ramparts with old cannons, blue fishing boats and fresh fish. The catch is the drive, about 2 hours 45 minutes each way, so bring snacks and plan a stop.
In more detail
Essaouira suits families with very young children who need downtime between sights. The pace drops after Marrakech, the air is cooler, and the medina is small enough to wander without a plan. It is very walkable with a carrier, though noticeably windier than the city.
There is plenty for children to look at. On the way, our trip stops at a women's argan cooperative with the famous goats in the trees. In town, blue fishing boats unload at the harbour, and the Skala sea ramparts have brass cannons and Atlantic spray below. The harbour lunch of grilled fish is an optional add-on.
The drive is the trade-off: about 2 hours 45 minutes each way across the argan plains, so the day starts with a morning pickup and ends back at your riad in the evening. A private car lets you stop for a break whenever a child needs one, and we arrange child seats on request.
Bring a windproof layer for everyone, whatever the season, plus sun hats and sun cream. In summer, Essaouira's Atlantic breeze makes it a welcome escape from inland heat.