We've been to Jiko a few times with young kids. My son went first when he was 6, and most recently at age 9. We're going back this summer, when he'll be 11.
He really enjoys it. He was a little skeptical at first, and like most 6 years olds, he was pretty nervous to try anything new. They had many basic "kid friendly" options, like chicken strips and macaroni and cheese, on the kids menu. His first couple trips he stuck with that. Last trip, he was more willing to try our entrees (my wife loves the scallops, I always get the chermoula chicken).
Since that time, he's become a bit more of a gourmand (thanks to my constant cooking experiments), and he now enjoys lots of fish, steak (rare) and anything I make in my Morroccan tagine. He's really looking forward to going this summer, and I'm sure he'll want an "adult" entree of his own.
The kids in our travelling group always love a trip to Jiko, though, for the dessert: They have a great "make your own S'mores" dessert that the kids just love.