How serious is it for a pregnant woman to be in contact with a child with chicken pox?

Can be serious. If the mom was not vaccinated as a youth or never had chicken pox it can be very harmful to the baby depending on what trimester.
Chickenpox & Preg. If chickenpox develops during the first 20 weeks of pregnancy — particularly between weeks 8 and 20 — the baby faces a slight risk of a rare group of serious birth defects known as congenital varicella syndrome.The second window of vulnerability is at the end of pregnancy. If chickenpox develops during the few days before delivery, the baby may be born with a potentially life-threatening infection.