Hi LK,
I love that you want to teach your daughter about giving. I think books that show characters being kind, fair and compassionate to each other is a great place to start. At this age, she is just beginning to explore her world, and we want to give her as positive of a worldview as possible. Cognitively, her brain is still on the present and cannot understand the whole haves/have nots/ those less fortunate concepts. Nonetheless, there are some things you can do together year round that encourage the good feeling that giving of ourselves can elicit:
Bake a batch of sugar cookies and let her decorate them with sprinkles. Let her know that some will be for home and some will go to the neighbors/friend's houses. Let her be the person to hand over the plate or bowl of cookies to the friend.
Do you have a neighbor who needs help with raking those autumn leaves? This is an act of kindness--helping--and your daughter can help to put those leaves in a bucket that can then be dumped into your yard debris bin or bags.
Lots of 'thank you's' at home. "Thank you for handing me your fork. It helps me when you clear your dishes." "Thank you for bringing your clothes to the basket." etc. This is an opportunity to appreciate the deed, so don't go far out on praise ('you're such a good girl') instead, point out *why* her action was helpful.
When she's with friends and playing, ask her if she can 'help me find another one (toy) for your friend". Help her notice how happy her friend is when they are given the toy--if that's the case.
Modeling is another very positive way to encourage thoughtfulness. My son and I recently were given a bag of day-olds at the bagel shop. We passed a homeless family a bit later and I gave them most of the bag of bagels. When my son (who is three) asked why, I simply told them "well, they were hungry and we had extra". He was satisfied and we went on our way. It's important to keep it simple, too.
We also model, too, in how we treat those around us at home.
One thing to keep in mind is that, when we are volunteering and out in the world, our children may not understand all that they see. Older children are better able to understand why an impacted person may be acting out in some way that may not necessarily be dangerous, but is attention-getting and disconcerting. I have found that serving in shelters and soup kitchens is very rewarding work for me, and I have also decided not to bring my son along until he is 12 or 13 and can digest all that happens in these environments.
Best wishes to you and yours. :)