First off, you can take a carseat on the plane for your child. There are NO rules against it. If you have a seat for that child, you can use the car seat. See my other post.
Secondly, there are extra oxygen masks in every row of seats on the airplane! They are very easy to put on...honestly. And you would have help. There is always someone there to help you.
Having a child on your lap is not a hinderance. During Flight Attendant training, we went through every scenario and having a child on your lap was not a hindrance and the child was safer in your lap than sitting in a seat next to you...especially one that small.
There are fewer plane crashes than you realize. The media just sensationalizes them. You are safer on a plane than you are crossing the street!
Have a safe and enjoyable flight!