I did travel once with my kids already in LH J (age was 1 & 4 at the time).
In my opinion it is better, when getting out of the plane after 11h they were ok. They can sleep, get more and better food and have some place to play.
I don't really understand why kids in J should behave better than in Y? The parents should always try to achieve good behaviour. I can't remember exactly, but I'm sure the youngest one cried a few times. That is just what kids of that age do.
90% of my time I travel for work so alone and several times I have seen kids/infants/toddlers in J. They sometimes make noise, but my worst experiences are always when two colleagues travel together in J and start drinking… This happens a lot and is noisy for a big part of the flight followed by wonderful snoring in the quiet A350.