I totally agree with Simon.
I have travelled with my niece who was 4yo on biz, nephew, 4mo old together with his mother and they didn’t didn’t disturb a soul. Children generally are glued on to cartoons or their own toys and games as well as the “aircraft’s childrens pack”.
Majority of my family, extended family and friends all travel frequently and travel with children in both biz and first class depending on route with a maid in economy. I have never encountered “spoilt brats running around in biz or first class”.
Mostly I find that it’s the adults that make a big fuss. Those who really pay for their fares know how to behave “ethically” and do not complain as much. I am blessed enough to be one of them and I have travelled on economy as well on short hauls and sometimes just to walk around to see how others behave!