I would urge you to take your children to Sunday school as soon as you can. It's good for them to grow up feeling comfortable and "at home" around church, and I believe you will be making an investment in their lives that could pay "spiritual dividends" for years to come.
And don't worry about how much your children will understand about God at this point. No, they won't understand everything about God; neither do we! But they may learn more than you realize and above all, they can understand that God is real (even if they can't see Him), and that He loves them and takes care of them. The Bible says, "Train a child in the way he should go, and when he is old he will not turn from it" (Proverbs 22:6).
At the same time, don't forget that you and your husband are the most important influence on your children's lives including spiritually. For example, do they sense that prayer is important to you, by seeing you pray with them before meals or before they go to sleep? And do they sense that Christ is important to you, and so is your church?
Make sure Christ is first in your lives and in your home. Then ask Him to help you raise your children so they grow up knowing that God loves them, and Christ wants to come into their lives. It's the most important legacy you can give them.