I would adapt the lesson from Primary 5, lesson 26.
You could use the enrichment activity #2 and adapt it to fit your needs.

You could begin with the attention activity, which is really great:

Show several keys of different sizes or shapes.
 

• Why do we use keys?
• What do you think these keys will open?
• What would happen if we lost a key to something that was locked?

When we speak of priesthood keys, we mean the power and authority of God, given to the prophet and other priesthood leaders.

Then show a picture of a temple and explain that the priesthood power is used in the temple to give blessings and baptize and seal families together.

Sing: "I love to see the Temple." (or another song)

Then briefly tell the story of Moses. (Use pictures or flannel board figures or have a child wear a simple costume.)

• Explain that Moses was given the priesthood power by God.
• Explain that Moses appeared to Joseph Smith in the Kirtland Temple (show a picture) and gave him the keys he held to gather Israel. Now millions of people in the world are being gathered to the Church by missionaries.

Sing a song like "I hope they call me on a mission" or another song the children know pertaining to sharing the gospel.

Briefly tell the story of Abraham. (Use appropriate pictures or props again)

• God gave Abraham the responsibility to bring the blessings of the gospel to the whole world. Abraham had a son named Isaac, and Isaac had a son named Jacob.
• Jacob had 12 sons and they became the 12 tribes of Israel.
• Everyone belongs to one of these tribes.
• Explain that Elias appeared in the Kirtland temple and gave Joseph Smith the keys to the blessings of Abraham.
• Since we are also members of Abraham's family, we want to help everyone on the earth be baptized and sealed in the temple. Then we can be with all of these people, (who are also members of our family too) forever.

Briefly tell about Elijah. (Use picture or prop)

• He was a prophet who lived in Israel during Old Testament times.
• God told Elijah that whatever he sealed on earth would also be sealed in heaven. He was the very last prophet on the earth to hold the keys of the sealing power of the Melchizedek Priesthood before the time of Jesus Christ.
• Explain that Elijah appeared to Joseph Smith in the Kirtland Temple and restored the
• priesthood keys of the sealing power to him.
• Because these keys were restored, righteous families can live together forever.

Bear your testimony and briefly review the facts that Joseph Smith (show a picture) was given the priesthood keys from Moses, Abraham, and Elijah, so that we can share the gospel with the whole world, and share temple blessings with the whole world (living and dead) and be together forever with our families.

All these things are possible because of the priesthood power.
(The majority of this information was adapted form Primary 5 lesson 26, found on page 143 of Primary 5 manual.)