Chickenpox or varicella is about the most contagious viral illness known in humans.  It is spread by droplets which seem to be able to get through keyholes, and is most contagious the day before the patient breaks out in pox.  Incubation in  the host is 14-21 days.
If mother has had it previously, her baby will have some degree of passive immune protection from it.  This period of protection is extended by breastfeeding.  As we have seen from the anecdotes, this varies a great deal.  It is consistent with this theory that children could get varicella twice-- the first time it is the passive immunity which gets them through the infection, but their own immune system doesn't respond, having been prevented from mounting a full-scale response by the presence of maternal antibodies.
Chickenpox is a trivial illness for people with normal immune systems.  It can be life-threatening to mothers in late pregnancy, and if mother breaks out in pox less than 48 hours before she gives birth or less than 5 days afterwards, the baby is also at high risk.  These mothers have had no antibodies previously with which to protect their babies, and the babies are so young that their own immune systems are not always capable of coping.  Where I live these babies are given immunoglobulin by injection as soon as the risk is known.
BTW, I actively tried to get my children infected with varicella when they were seven, and one and a half, because my eldest had juvenile rheumatoid arthritis which was being treated with aspirin.  She had to stop the aspirin whenever chickenpox cropped up because of the risk of Reye's syndrome, so I wanted to get it over with.  The children and I visited every home with chickenpox in our acquaintanceship from Easter to August before succeeding, and she was over the illness just in time to start school.  But it turned out the arthritis had settled down so much by then that she has never resumed the aspirin treatment after that 4 month trial period off it in 1988!
Rachel Myr
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Kristiansand, Norway

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