The doll's house is a symbol of class division. It is so because it is a unique gift which is expensive, detailed, and mostly appreciated by those who can afford such a gift.
The importance lies on the fact that the Kelveys, as opposed to the other girls at the school, were specifically excluded from even coming close to the Burnell's household, even if it is to look at the house up close. This is because the Kelveys had no known father, their mother was a washer woman, and their poverty makes them ill fit in relation to the other girls.
When the Kelveys accept the invitation to see the doll's house, albeit reluctantly, they are given a once-in-a-lifetime chance to see something that only the fortunate can see. In fact, they get to experience a small part of the world as the Burnells know it. Hence, the importance is primarily that the doll's house represents the opportunity that the Kelvey girls had, for once in their lives, to aim at something higher than what they knew. Having had a glimpse of the ornamental lamp is a symbol of having an equal opportunity, even though such opportunity may never manifest itself again in the life of the poor girls.