As most have said Chickenpox is at it's most contagious 2 days prior to spots appearing until they have scabbed over.
In your case if it
IS Chickenpox you'll be okay as your child will be well on the road to recovery and all spots should be scabbed and dry by then but if anyone else is in a similar predicament and planning on hiding spots I would urge extreme caution.
Chickenpox can be especially harmful to unborn babies, elderly and anyone with low immunity so to deliberately take a risk with someone elses health is iresponsible in addition to which a child with Chcikenpox needs to be kept cool to help stop itching and infection so going to sunny climates isn't a good idea in the first place.
At the end of the day IF they won't let you travel you CAN get a full refund and can book another holiday - not ideal and very disappointing I'm sure but surely the responsible way to deal with things.