When the house is ready. Wendy says, " What a lovely little house! Thank you."

当房子盖好时，温蒂说：“多么可爱的小房子啊！谢谢你们！”

The past perfect sentence emphasises that the action is complete so I've finished cleaning and as a result the house is ready for the party.

过去完成时的句子强调动作已完成，因此我已经完成了清洁工作，所以房子已经为聚会准备好了。