Children can be chief; I have a 7-year old chief, once, alternating between chiefly duties and playing in the dirt.

Nursing mothers is probably it. Either the mother or the child is probably chief. To make sure you don't miss anybody, it's easiest to pause the game, then drag everybody on top of the robe at once, and unpause it. Another, more meticulous method, is to use the detail screen to try each villager one by one.