Find a church that has children's programs. Realize that if you want your kids to understand about Christmas, the once a year thing won't really do it. If you got turned off from Christianity becuase of the church or religion you were involved in try another church. Some churches are spirti filled and have a lot of joy - others do not. Even the church I grew up in is vastly different than if was when I was younger - they are vitally involved in kids and teens and have programs so that the kids aren't sitting through a boring church service geared to adults - but instead are in their own part of the church stomping, dancing, making crafts, playing games that relate to biblical virtues like individuality, service, patience, helpfulness, generosity, friendship, etc.
As for volunteering - most of that takes place with adults. Do you really want to even think of dropping your 7 yr old at a food pantry by himself? Be an example and do it yourself and bring your son along.
Another idea - I'm sure there are elderly folks in your neighborhood - bake some cookies and go with your child to the door to deliver them. You'll be surprised at how welcoming seniors are - so many of them are lonely as lose their friends and the ability to get out and visit with eachother. They are starving for company. The middle school youth group at our church goes leaf-raking to the homes of seniors in our area. They are always delighted.
Enjoy this time together - you're a great mom for wanting to instill these values early in life.