It's also worth remembering that an airplane is a very good environment for the transmission of germs - it's enclosed and the air is simply recirculated, and you are all cooped up together on there for the length of the flight plus all the time on the ground too - which is probably worse than 15 minutes waiting in a doctor's surgery, or being in the same supermarket as someone with chickenpox.
Just to put this in context, if a woman who is either 13-20 weeks pregnant, or 36+ weeks pregnant catches chicken pox there is a small chance that the baby can develop foetal varicella syndrome, meaning the baby is born with serious abnormalities such as scars, eye problems, and shortened limbs, and may also have neurological problems causing bowel and bladder problems and also possible developmental delays.
The thing is that you know that it's highly likely that your child will be incubating chickenpox when you take them on the plane, so you know that will be a risk to anyone immunosuppressed or pregnant who happens to be on that flight - and whilst you have a choice as to whether to take your child on the plane or not, they will have no choice as to whether they have to spend a couple of hours shut up in a metal box with your child's very contagious germs.
It might be fine, there might be no-one immunosuppressed or pregnant on the flight, but even then you could blight someone else's holiday by giving them chickenpox - and if it's an adult, believe me, chickenpox is a nasty disease to get when you're grown up - I should know, I got it when I was 25, and was quite poorly with it.
I don't think it is right to take these sorts of risks with other people's health.