I ate at Jiko last April with DS and his family. My Grandchildren were 3 and 5 and are very used to restaurant dining and "the rules". They were very well behaved. They were happy with the Children's Menu. They had salad and one had Mac and Cheese and one had the fish which happened to be Salmon. They loved it.
I ate at Jiko last Friday evening with DH and other DS. We were a party of 3 adults. One thing I noticed was that there were virtually no children there. It was a very adult atmosphere. It was quiet, elegant and a special meal.
Kids are welcome at all restaurants at Disney, (with the exception of Victoria and Alberts), but there just weren't many at Jiko last week. To be honest, as a party of 3 adults, it was really nice to have an adult, refined meal (I have been on both sides of this issue)
HTH