Were the child's pox scabbed over? If so, the disease was no longer communicable and there was no reason not to travel.

As has already been mentioned, varicella infections are communicable for several days prior to the appearance of a recognizable rash, and it takes two to three weeks for the illness to manifest after contracting the virus. That's why entire classrooms tended to experience outbreaks all at once before the introduction of the vaccine.

Chickenpox infections that never develop into full-blown rashes are also not uncommon, but they still follow the normal stages of contagion.

Varicella virus is also transmitted by people with active shingles cases. Shingles lesions can occur on any part of the body (e.g., under clothing), and even internally, so you will often have no way of knowing you've touched or been near a person with a contagious case of shingles.

In other words, you probably encounter many opportunities throughout the year to contract the virus without even knowing you've been exposed. You yourself could be one of those people who have actually been infected in the past without anybody ever realizing it. The only way to be sure is a titer test.

The initial varicella infection (chickenpox) is indeed significantly more dangerous in adults than in otherwise healthy young kids. Regardless of whether this child was actually fit to fly, you should either be tested or obtain the vaccine.