C.C.
If it were my daughter and my mom, I would definitely send her - but that's my family. My parents have been taking my kids for a week-long camping trip every summer since my older daughter was 2, and it has been wonderful for all involved. I guess the question is, do you think your mom can handle it (is she young and fit and patient enough to chase a preschooler around for a week?) and do you think your daughter can handle it (is she pretty independent, or is she extremely attached to you? Does she spend a lot of time with Grandma?). What helped me decide to send my kids on vacation with my parents was - my parents raised me to adulthood, and they always kept a close eye on me. They respect (and heavily influence) my parenting values, and they love my kids. Sure, I miss my kids during the week they're gone every year, but as my mom says, "How can you miss them if they never go away?"
But every family is different! Good luck with your decision!