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I think that I would set it up when they come, and have her sleep in it after they leave. My main thinking is because what if she really likes it, and then she has to get kicked out of it so her grandparents can sleep in it. I would keep her in the toddler bed; then when you go to put her in the full size I would make a big deal out of going to the store and letting her pick out her own new sheets or a bedding set, something that she will be really excited to use but has to stay on her big girl bed.