The Green Glass Sea by Ellen Klages 321 p.

Dewey Kerrigan's mathematician dad and other scientists are working secretly at Los Alamos, developing a "gadget" they hope will end World War II. Dewey, small for her age and with a limp, is considered a bit odd by the other kids at the site, since Dewey loves to build mechanical devices with castoffs from the dump. She has trouble at the top-secret compound until she comes to know Suze, another misfit.