As the younger Burnell child, Kezia knows how it feels to be excluded from decisions and acceptance. While the older Isabel is allowed to determine all the rules governing the display of the dollhouse to the girls at school, Kezia and her sister Lottie are left to follow helplessly because "Isabel was bossy, but she was always right, and Lottie and Kezia knew too well the powers that went with being eldest."
It is Kezia who sees the Kelvey girls always being excluded from the other girls at school. It is Kezia who asks their mother if they can invite the Kelvey girls to see the dollhouse as well. And it is Kezia who doesn't follow her sisters on the afternoon when she invites the Kelveys to see their magnificent dollhouse; perhaps she knew that Lil and Else would be passing their way and had plotted a means for them to enjoy the house as well.
While Isabel desires the fame and adoration of all the girls at school and uses the dollhouse to secure a prominent social standing among them, Kezia sees two girls who are excluded from the rest of the group and seeks to include them. She wishes for them to experience the same grandeur, even if for a moment, as everyone else. She takes them to the house in private, ignoring both her mother and sister's wishes in excluding the Kelveys.
Isabel's motives are self-serving, but Kezia's motives are genuine and focused on helping others.