The fact that you aren't sure how, means you should not "come out", at least not in one fell swoop. You don't know their reaction, and it could easily backfire.
Instead, make sure that they know you as a person of concience. IOW, show them that you care about and take a strong stand on important things in life, but that you also respect letting other people (like your kids) chart their course in life. Hint: getting 10 hours a month in service is not an important thing in life; getting a good education, having an inquisitive mind, being active in making the world a better place, treating people as equals (men, women, racial and sexual minorities, etc etc), appreciating art and music, having hobbies that you love.... these sorts of things are important.
If you see a neat looking volkswagon bug on the streett tell your daughter that you think it fits her personality and you think she'll probably end up driving off to college in a bright yellow bug. That sort of thing; you could probably very organically find 10 subtly subversive topics per day if you keep on the lookout for opportunities to informally witness ;)
See how easy it is to provide a better alternative to the witnesses?
You know how the lame shit your ex-husband does and says makes you roll your eyes? The trick is to get your kids rolling their eyes at their dad's religiously inspired lameness along with you. Don't let them see your anger, but do let them know that you aren't distancing yourself from "the truth" because you are lazy, or just want to do your own thing, but rather because you don't see it as "the truth".