It is pleasing to John to see the growth of believers in the church to which he is writing. By using the word "greatly," he emphasizes the magnitude of his happiness. He had either heard this news or, most likely, had personally encountered it. He was especially joyous when he saw children walking in obedience to the gospel. Following that, John explains that obedience has a source, which is God's command.
The truth is that Christ died for our sins, was buried, and was raised on the third day, as stated in the Scriptures. The truth is that God loved the world so much. As a disciple of Jesus, John understood how crucial it is to know the truth, to safeguard the truth, to preserve the truth, and to live the truth in our everyday lives. 
The one who walks in truth is not only doctrinally sound but also humble, growing in grace, rejoicing in the Lord, and living in unity with their brothers and sisters in Christ. May we walk in line with God's Word and obey his commandments.

Dear Heavenly Father, it is a blessing to read this small piece of second John, which reminds us of the importance of walking in God's holy Word and loving one another as Christ loved us and sacrificed for us. Please help me to continue walking in truth and love, as Christ commanded. Please let my life be a living testimony of Your goodness and grace for Your greater glory. 
In Jesus's name I pray. Amen.