Have them go through their toys and clothes, choose things that are in good or "gently used" condition, and find a place to donate them. There are shelters for battered women (which you cannot visit) but many agencies that serve foster kids, homeless families, new immigrants and others who are either in temporary shelter or just getting their own new homes. They can use these items, and often you can take your kids right to the location to help. This is a great exercise before they get into the "new stuff" syndrome of the holidays!
We have an agency call the Interfaith Hospitality Network which houses families in transition from homelessness to homes. Different churches and synagogues take turns housing and feeding them. Sign up for a night, and work with your kids to prepare a meal at home which you then deliver. It's a good way for the kids to learn about nutrition and budgets as well as learn the skills needed to prepare a meal. They can help serve, and they can play with the guest kids to help distract them from their situation. Showing warmth and hospitality is important.
Find a nursing home that would like to have your kids come in to color pictures, make snowflakes or turkeys or other holiday/seasonal decorations with some of the residents. You can request that your kids work with a higher functioning group so they aren't frightened by too much medical equipment. An Alzheimer's unit often has higher functioning people who relate well to kids - sing songs or play gin rummy or any other activity the staff recommends.
Collect food for the local food pantry. Organize a drive in your neighborhood and have your kids create and distribute flyers, put a collection box outside your garage or on your porch, or go pick up food at a pre-determined time. Start a "change jar" with a big clear canister and ask your neighbors to clean out their car and couch cushions, or take all the coins from their wallets. It's a small, manageable amount from each family. Ask a local bank to sort the coins for your for free since it's being donated to a cause.
There are many food drives now, so consider doing another food drive in March or May when people aren't thinking charitably. Summer is the worst time for food pantries because everyone gets in "vacation mode", and many many children are home from school without a subsidized or free lunch program. If you put something on your calendar for every month, it will reach a lot of people and connect in your children's minds that charity is not just for Christmas, it's part of their lives.
Our son always had to take some of his money (birthday money, chore money, whatever) and give it to charity as well as savings, on top of what he wanted to spend on his fad of the moment. Make it a habit and it becomes second nature.