First, I commend you for your dedication and your willingness to undertake this responsibility. What would happen to the people you help every day if you and others like you didn't assist them?
One of the marks of a true follower of Jesus is love; as Jesus said, "By this all men will know that you are my disciples, if you love one another" (John 13:35). And one of the characteristics of true love is a willingness to help people who can't give us anything in return. Every person you serve–no matter how disabled they may be–is the object of God's unchanging love, and God calls us to love them the same way He does.
No, those you see every day may not be able to understand very much about God. But they can sense God's love for them by seeing it in your life. Sometimes we serve God best by our smile, or our concern, or our encouragement of someone who is facing great difficulty. Jesus said, "You are the light of the world. ... let your light shine before men, that they may see your good deeds and praise your Father in heaven" (Matthew 5:14,16).
God knows all about those you serve every day, and they are the objects of His mercy and grace. Ask God to help you share God's love with them in whatever ways you can, and to point them to the hope we have in Christ.