When Alice Brooks and Bettina Chen met in 2010, both were in engineering master's programs at Stanford University—mechanical and electrical, respectively. But there weren't many other women around.

Chatting about why there were so few female engineers, the pair realized that they had both grown up with toys that encouraged them to build and make things, rather than traditional toys for girls. Ms. Brooks, now 26, received a saw for Christmas at the age of 8; Ms. Chen, now 25, had similar experiences with do-it-yourself playthings....