I suggest you go over to
www.biddingfortravel.com and read the boards on flights from your area.
I am not sure about the lap child, but I think there is a risk that you won't be seated near the rest of your party. I think that you don't get seating assigned by PL when you book!
From PL.com
According to airline policy, an infant is a person under 24 months of age (at the time of both the departure and return flights). If you're planning to bring an infant on your next trip, here are some things to keep in mind:
Flights within the U.S. and U.S. Territories:
If your infant is going to sit on your lap, you are not required to purchase a ticket for him/her. Keep in mind that the airlines will only allow one lap infant per adult traveler (age 18 and above) so you'll need to buy a ticket for each infant exceeding the number of adult travelers.
If you do not want your child to sit on your lap, you will need to purchase a separate ticket, and you will be required to seat the infant in an infant car seat during the flight. You are responsible for providing the infant car seat.
If your child will be under 24 months of age when you depart, but will be over 24 months of age by the time you return, you must purchase a ticket for him/her.