uklady on Family Tree Circles

Hi... I am researching my partners family tree... BANHAM.. His G.Grandfather as Walter Banham..Born in Thetford Nofolk..but settled in Bromley Kent UK..The cenus of 1891 states he was married with one child "Lily Banham"...come the 1901census he now has 2 children...Lily and Harold..his Grandfather...The promblem arises..from the fact that Lily Banham, has never been mentioned either by his G.Grandfather, Grandfather.. or Father... So where did she go and what happened to her? If there is anyone out there who could help I would be greatful. Thank you...