We have been flying with our now 6-year old daughter in business both on QR and AY (first time when she was 3) and have been very happy with every flight. She has been able to sleep well on night flights, been treated extremely well by the cabin crew, been given surprisingly tasty kid's meals (remember to order those in advance) and generally has enjoyed the flights.
I was a bit nervous before the first flight if the distance between seats was too much but didn't have any problems. If flying on AY though it's easier to help the child with meals etc when flying on A330. So, if you have a chance to fly in J with your child, it's definitely worth the extra cost.