Ear plugs work wonders!
I have travelled long haul with 2 ( now thank fully teenagers)The oldest was 6 weeks on their first long haul flight.
I blame the parents and that included my Ex who did not get that you can not just get on and relax if you have young kids.
Make friends with the cabin crew, tell them what you need and roughly when always after food service - never during!
If they are awake, you are awake, if they are asleep you are asleep. Rules are set and enforced, no screaming, no crying unless you broke a bone, no fighting no biting and no kicking seats - all had immediate consequences ie, treat box had something removed - ie chocolate. They were given lots to drink, so had to get up and go to the toilet regularly, when up they could walk up and down the aisle and come back, sit down and watch the ipad, read or we would quietly talk, read a story etc as age appropriate. They carried their own rucksacks which they put things in they wanted. Soft toy and favourite small plane blanket.Always went for the row at the back as more space and less people to annoy.
There is no healthy eating on a long haul flight. They both got a treat box, exactly the same and they had to make it last - amazing how they work it out quickly. Long haul to NZ twice per year takes a military operation with 2 under 3!
When babies always start drinking half an hour out on the descent, lots of cuddles, stretches and walking, I loathed dummies but they were essential on planes till 1 yrs old.
Once one child cried on descent as a baby, two occasions of puking and never had a complaint.
Now as teenagers they comment on badly behaved children who do not know the "rules" and say the parents are soft!
So much is down to the parents