Since this is your first flight I want to congratulate you on looking into this matter. Your 2 yo is free on most airlines for sitting on your lap...this is a hot topic that tends to get out of control. Some airlines off a discount for a child 2 paying for a seat. Finally, I have flown enough that I would never put a child on my lap the whole flight...I know it is an extra expense, but think of it this way the 2yo doesn't need a ticket at WDW, and will usually be eating off your plate(or is free at buffets). So this is the one place I would pay for the comfort and saftey of my child. If the cost is prohibitive look into SWA or other lower cost airline.