Yeast is a fungus that converts sugar and starch into carbon dioxide bubbles. It is what makes bread rise and dough stretch. It is the leaven that Jesus referred to in the gospel. Only, Jesus is warning against temptation and sin that compounds and grows inside our soul. And like children not yet mature, the disciples still didn't understand.
One more thing: Like all good parents, Jesus is constantly teaching. And as every parent knows, some lessons have to be repeated over and over again. Use the bread analogy from today's gospel to teach your children about temptation and sin. Then reassure them that God does not tempt us. Rather, He gives us every good and perfect gift.
Prayer: Father in heaven, thank you for every good and perfect gift you have given me, especially my children. Lead me not into temptation. Let me never be parted from you. St. Juliana, pray for me. Amen.