I also involve them in my spiritual life. I pray my big, serious in-depth scripture-using prayers with them and soon they start to do the same. I tell them sometimes when I mess up and God disciplines me (they think it’s funny that God “spanks” mommy). I teach them the things God has taught me, using a lot of personal stories. A book I read that helped me to think about how to grow my kids spiritually was Transforming Kids into Spiritual Champions (something like that). I recommend it.
Sometimes I think we can water down spiritual truths for kids because we don’t think they can comprehend it, when in fact they can. So invite them to observe and try grown-up Christianity, if you will. For instance, I have our kids fast through lunch on Good Friday. We also re-enact the events of Easter week using play-doh and pipe cleaners (I make them into men who represent the disciples, Jesus, crowds, the angels, the Romans, etc. We read a section of the story each day on the day it happened in the week leading up to Easter, and then we all play parts in the story and act it out with the pipe cleaners).
I also let them watch grown-up Christian movies, fast forwarding through any parts that are not age appropriate. Like we watch the Nativity Story at Christmas to emphasize what Christmas is about. I also get them involved in serving or giving at Christmas. One year we fed and gave gifts to the homeless. Another year I let them pick poor entrepreneurs in third world countries to support.