I would say the best way is by example when they are with you and through simple child friendly teaching. Herald Kids makes DVDs about the faith that are simple single message. My five year old loves them. He told me today that Brother Francis (cartoon character) says that sin is like dirt in your heart.
I know you have probably thought of this but could you change your work? Kids at that age especially boys really need dad time or hire a child minder in your area? It must be so hard to be away from them.
Hang in there!!