I found this on the internet and it seems to fit your situation:
Q. I'm flying to Antigua next week with both my children. However the oldest Zoe, who is 3 has come into contact with another child who has since developed chicken pox. What are the chances she will get it too, and will she be allowed to fly if she does have it?
A. This is a question that I am being frequently asked at the moment as we are right in the middle of chickenpox season.
I understand your concern as it is impossible to tell if Zoe will contract it from her friend and when it will come. You obviously don't want to cancel your holiday only to find that she remains in perfect health, and neither do you want to turn up at the check in desk just as the spots erupt to be told you cannot board.
There is a lot of conflicting information around but for your circumstances this is what I advise.
The incubation period for chickenpox can be anything from 7 to 20 days. So you can have a long wait to see if Zoe is going to develop the illness. However , during this incubation she is not contagious until she is about to come up with the rash. You can tell this is about to happen as she will develop symptoms like that of a cold. She will be snuffly, may have a fever and be generally unwell and clingy. If this happens the chances are that she is about to erupt in blister like viral filled vesicles, and, as she will be infectious you should cancel the flight.
If she remains well over the next week then she is unlikely to be incubating the illness and you should go ahead with the holiday as planned. Of course there is a chance that with a maximum of 20 day incubation that she could go down with the illness whilst you are in Antigua.
Once she has the spots then she will be infectious to others until the very last spot has finally crusted over and fallen off and should not fly.
Chickenpox is caused by a variant of the Herpes virus, and is spread by respiratory droplets, so if she coughs or sneezes then she can infect anyone in the vicinity.
This creates the difficulty with flying as air is recircrculated around the plane so others may breathe it. Therefore there is a danger that any passenger may contract the disease. This can be a disaster if anyone on board has AIDS or is immunosupressed after a transplant as chickenpox can be fatal in these people.
So if Zoe stays well this week feel free to board the plane and enjoy your holiday, but if she begins to fall ill then cancel as soon as you can. This will be reimbursed by your insurance policy.http://www.e-med.co.uk/travel_health_questions/fit_to_travel...