Thats why airlines recommend a car seat when purchasing a seat for an under 2, as CARES has a minimum weight of 22lbs and should not be used if the child isn't ready.
How far a FA will go to enforce this is up to the individual crew, but I think it's helpful know the policy in advance in order to best plan for any scenario.
From AA for example:
Reserved seat
If your infant will travel in their own seat, you must buy a ticket. The infant must either travel in a safety seat approved by the Federal Aviation Administration (FAA) or be able to sit upright in their seat without assistance and have their seatbelt securely fastened during taxi, takeoff, landing and whenever the 'fasten seatbelt' sign is on.
Traveling with children − Travel information − American Airlines (aa.com)