There’s a Dragon in My Backpack is one book in a series of adventures about a boy named Eric who has a pet mini-dragon. I would recommend this book as a good transition from picture books to chapter books for ages 6-9 as it was relatively entertaining and exciting. It also had an excellent example of jealousy, which is common in the age range and gives you an opening to discuss envy.
Eric has a neighbor boy about the same age. Unfortunately, they are not friends as the neighbor is easily unlikeable. One of his traits is caring about material items. He cares about the way people dress, the toys they have, etc. His attitude is these things matter and defines you as a person. There is a ranking of objects, and the nicer (i.e., more expensive) equates to you being a better human. He gets jealous of Eric for having a fancy backpack which he steals. The mini-dragon was in the backpack, where the central conflict/adventure begins in the book.
Exodus 20:17 You shall not covet your neighbor’s house. You shall not covet your neighbor’s wife, his male or female slave, his ox or donkey, or anything that belongs to your neighbor.
Exodus 20:17 is the 9th and 10th Commandments. Note: I will use the Catholic numeration for the commandments in this post. These two commandments are unique in that they discuss desire as opposed to actions. Thus, God not only wants us to be holy in our actions but be transformed on the inside, and He knows the heart (Acts 15:8)

Discussion with child: The desire for material things is not always evil. One may desire a backpack because it is necessary to carry many items, and it frees the hands. This is a reasonable desire. However, “often [our desires] exceed the limits of reason and drive us to covet unjustly what is not ours and belongs to another or is owed to him” (CCC2535).
The neighbor boy already had a backpack, so his desire for Eric’s “exceeded the limit of reason.” This envy (or greed for the ‘better’ backpack) moved him to action to also break the 7th Commandment: You shall not steal: Ex 20:15.
One of the things I do to combat envy is to focus on gratitude for what I do have. For example, suppose I envied another’s backpack. In that case, I give thanks to God for the one I have and focus on its ability to perform a function, and not on how it looks or how much it cost: “Thank you God for giving me a backpack that is sufficient in carrying all my belongings.” Likewise, if I am jealous of someone else’s clothing, “Thank you God for all the clothes I have in my closet that cloth me and keep me warm every day.
The Catechism of the Catholic Church (2540) gives us advice on how to combat envy – by praising God:
Would you like to see God glorified by you? Then rejoice in your brother's progress, and you will immediately give glory to God. Because his servant could conquer envy by rejoicing in the merits of others, God will be praised.
I do this when I find myself jealous of talents I don’t have. For example, I’m not a good public speaker, and sometimes when I listen to others give a speech, I find myself having envy. I first remember that God has given us each different gifts (Rom 12:6). I thank God for giving them the talent of public speaking so I can benefit from them and be grateful for the skills that I do have. At least for me, gratitude for what I have helps the most in conquering envy.
Finally, what if you don’t have something that you need. For example, what if you can’t afford a car and see many families with more vehicles than people with a license? In those cases, you have to go to God in your need. He will eventually give you an answer to your needs. It most likely won’t be how you envisioned. First – it most likely won’t be on your time frame. Be patient. Second – you may be given a beater of a car or someone to drive you as opposed to a car, but whatever the answer – be grateful and give thanks and praise to God.