I am in two minds about this.
I think it’s good to involve children in workplace visits etc. However, ATC is a safety critical environment, and you are working with people who have never met you and might have been startled to hear a child’s voice; should they trust the instruction or not?
Even a split second can make all the difference in a critical situation; just have a listen to the great work ATC did when BA38 777 crashed at Heathrow two years’ ago. That was on a normal, quiet day, and then all Hell broke loose.
While an extreme example, I just don’t think people charged with safety should “take their eye off the ball” for even a second, and having considered it I think it was a poor decision on this controller’s part to let his young child communicate with active traffic.