The Burnell children see the doll's house primarily as a status symbol, an opportunity for them to show off how incredibly wealthy they are. Even at a young age, children are acutely conscious of economic differences; and if they're fortunate enough to come from affluent backgrounds they invariably want to let everyone see that they have the most expensive trainers, the latest smartphones, and so on.
It's the same with the Burnell sisters. Owning such a large, expensive doll's house immediately sets them apart from other children, reinforcing their snobbish sense of superiority over the poor, downtrodden Kelveys. The doll's house also gives Kezia Burnell the opportunity to defy parental authority, an opportunity that most children are only too happy to grab with both hands. Inviting the Kelvey sisters to look round her doll's house probably makes Kezia feel like a bit of a rebel. In any case, it would once again seem that the doll's house is being used for a purpose other than innocent play.