Take them to homeless shelters, battered women and children's centers, and other avenues to volunteer. You will have to be intentional about it - if you aren't then you won't escape the hoopla.
Another thing is give gifts to people who need help. Our church did a "gift catalogue" this year for all of our work in the inner-city, our city, and the around the world - you give so much money for a project or something for someone less fortunate. Don't just give money to charity and tell the kids - let them be a part of picking it out and getting involved.
I remember one year when my kids were small and things were tight they wanted to take some toys to a center for abused women and children. They wanted to get their old toys, so I told them - if they were at that center would they want their toys - or not? They went through them and realized some were like new that they had outgrown, others needed to be fixed and would be fine - and some needed to be chunked! What Nikki and I were doing was personalizing for our kids how the child would feel receiving the toy - often what we do is for us, not the one we are really supposed to be helping.