Simplify your lesson plan. Principles of flight are rarely taught in any detail to children as young as those in the second grade, so you will need to keep that in mind as you teach the lesson. Do not go into detail about the forces at work or the explanations and theories as to how they work. Stick to the basics.
Explain lift as the opposite of gravity. Review for your students that gravity is the force that pulls objects toward the earth, discovered by Isaac Newton. An opposite force is necessary to keep airplanes in the air; the opposite of gravity is lift. You can attribute the process of lift to changes in air speed and pressure that keep an airplane afloat, or go into further detail if you feel your students can handle it.
Explain aerodynamics as the movement of objects through air. Talk about this concept in terms of what shape moves through the air the easiest. Explain that airplane engineers study the way air moves around the different parts of the airplane and design the wings, tail, nose and body of the plane so that the plane can move easily through the air. Go into more detail if you think your students have grasped the explanation so far, but stick to the basics if they start to look confused.
Give a lot of examples as you explain these processes. If possible, use demonstrations as well. The more your students can apply what you're teaching them to their everyday life, or if they can see it happening while you explain what it is and why, the better they'll be able to understand these concepts. For example, liken aerodynamics to flying a kite and discuss lift while showing a video of a bird flying.