One lovely thing to consider about volunteer work is that it only matters if the child *chooses* to volunteer. And that the opportunity be age-appropriate to your child's level of understanding.
Several people have posted here in the past asking questions on this subject. I would look into some low-key community based projects, like growing seeds to start plants for families who would like to grow their own food but are not financially able to get started. Our city has a group called "Growing Gardens" that does this, and my son's preschool has the children planting seeds for this organization. The flats of plants are then taken away, after becoming beautiful, viable plants.
A neighborhood clean-up walk together is also good. She can wear kid gardening gloves and help you pick up paper trash and non-sharps refuse. Then you can talk about it later, how nice things look. If your neighborhood is tidy, hit a park and do the same.
Or perhaps you could organize a garage/bake sale with proceeds going to your local homeless shelter, or the victims of the earthquake/tsunami in Japan, or those devastated by the recent storms in the South. She could look through her toys and decide to donate some for the sale. She could help with the baking, which would be fun for her, and the sale, as she could. This is an all-around win/win idea, in my book.
It's my experience, however, that children don't always learn the lessons we want them to learn until they are older. Storybooks about the value of charity and kindness ("Mufaro's Beautiful Daughters" by John Steptoe is a great one for children this age, and any of the books by Demi) are very informative. We also want to help our children gently become aware of life's inequalities while not leaving them feeling helpless because they really can't fix it. I also caution against children volunteering at shelters/feeding unless you know the environment. I worked for a while with a mobile soup kitchen, as well as some other more stationary hunger projects, and there can be people affected with serious mental health issues. So, keep it safe and low-key, and let her grow into this understanding as she matures.