Jiko is probably the quietest of the signature restaurants that I've been to. It's not very kid friendly in atmosphere, but we did see a handful of well behaved older children when we were there on 3/27.
That being said, while dining a toddler started screaming/crying. Our table had enough time to just look up and wonder what was going on than our eyes found the table with the screaming child and we saw a manager walking over to the table to see if she could do anything to help.
The parents must have waved her off because she left and they continued to sit there with their screaming child for the next 5-10 minutes. We were sitting at the far end of the restaurant (away from them), but we could tell that they got quite a few looks from other patrons and when their server came over to see if they could help - they finally took the hint and walked with their child until they calmed down enough to eat peacefully.
I have been to other signatures with unruly children, but Jiko is the only place that I saw wait staff and management jump in to calm the situation immediately, which makes me think Jiko truly is more adult orientated.