My experience paxing with 3 kids in the last decade is that we got the seat belt extensions about half the time - the rest we had to hold the infant and hope for the best.
Youngest son also once travelled in a bassinet suspended from the overhead lockers on an Air France 747-400. For the return flight I was particularly keen that he be in a bulkhead bassinet, as the first one was just under an aircon outlet, and the cigarette smoke from the smokers was being recirculated and blasted onto him.
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