Use the sports analogy.
Democrats are "The Blue Team," Republicans are "The Red Team." During the primaries cheered on specific candidates, but always stressed the team aspect even if your chosen candidate didn't win a primary, it was A-OK, there was someone on "our team" ready to step in and take her or his place. The sports analogy is something that kids can readily grasps.
Sometimes it's challenging, but we try very hard not to make negative comments about candidates in front of our children. If you have daughters you should be careful about what you say about Hillary Clinton and Sarah Palin. However I feel about them, they are two women who strive to be president, and it's important for girls to see how very possible that is. Don't disparage candidates in front of your children. Talk about who is most in line with your personal values.
Tell them (and show them) that anyone can help get their candidate elected.
Take your kids to visit the local election headquarters. Stop in to pick up stickers or buttons and let your children chose one they like. While at the HQ your children see the people on the phones, they see the posters all over and they know how hard your neighbors are working to get your candidate elected. This may also encourage them to get more involved as they get older and not be afraid to stop in to headquarters for more information.