On which studies are you relying for this known in-utero x-ray / cancer connection? I think the findings of the Oxford Survey of Childhood Leukemia from the 1950s have not been easy to confirm. Check out:

http://www.jfponline.com/pdf%2F5505%2F5505JFP_ClinicalInquiries4.pdf

Every time you fly you get a dose of cosmic (gamma) radiation roughly equivalent to a chest X-ray. As carcinogens go, I wouldn’t worry about infrequent X-rays exams (as compared to alcohol, tobacco, common viruses [HPV], and plastics [vinyl chloride]).

Of course, there’s no reason to get an X-ray unless you or your kid really needs one.