K.H.
We flew from VA to Cancun in November with our 3 kids , aged 6 , 3 1/2 and 17 months at the time , the flight was 3 hrs and we didn't take car seats. I really don't think you need to take them on vacation with you unless you plan to drive at all , even when we do plan to drive we check them in as luggage and never take them on as hand luggage , it's just extra bulk that you don't need.
Take some food/snacks on the flight though because it is really expensive to buy the stuff they sell.