Recently I had the crossful opportunity to be with my daughter (4 years old) in the Emergency Room after a massive seizure. [She is alright now and back on medication, although more tests are coming in the next couple weeks at a Children's Hospital.] What humbled me and encouraged me is that while she was in the ER, unable to read (normally she can) while she was recovering from the seizure, she began to hum "A Mighty Fortress" (LSB 656 not 657). This comes from having heard it sung at church, but mainly from hearing it sung around our house and at bedtimes and so forth. It has been a major focus of my wife and I to not hold back when exposing our children to our hymns of the Faith.
The next day while recovering at home she would often break out into singing the common doxology. This too has been a standard "quick" bedtime song used by mom and dad when it is too late to do anything more (like tonight, staying in a motel outside of Des Moines, IA).
Something often taught in our churches is that these little ones need to sing the easier hymns (if hymns at all) and only be exposed to those things "age appropriate". My daughter's experience shows the wisdom of the ages, that a child's brain is like a sponge, soaking up things which it cannot yet understand, but will nonetheless absorb and repeat. Sometimes that is the very model of faith too – not being able to understand things at first, but simply taking them in and confessing them back in a childlike way (liturgy anyone?). Learn the words first, Luther would say, the meanings take a lifetime.
In your homes, don't be afraid to share those things of our Faith with even the littlest ones, you can know that their minds are absorbing it, and someday, even when the times seem dark and the crosses hard to bear, they may end up being great reminders of Christ that lighten the heavy load.