A lock and key for the attention activity (for example, a padlock, diary, jewelry box, or suitcase lock)
A cutout copy of a paper key with the word Covenant written on it
Display a key. Ask the children what a key is used for. You might have a child use the key to open a lock. Hold the paper key so the children cannot see the word Covenant, and tear it in half. Explain that a key would not work if you had only half of it. Turn the pieces of the key over to show the word Covenant. Ask the children if they know what this word means. Explain that in the gospel a covenant is a sacred agreement or promise between Heavenly Father and his children. When we keep our covenants with Heavenly Father, he will always fulfill his part. But if we do not keep our covenant promises, the covenant is broken. Keeping our baptismal covenants is a key to blessings and eternal life. Explain that eternal life is becoming like Heavenly Father and living with him forever. Explain that as the children learn of Alma and his people they will be reminded of their baptismal covenants and how they can better keep them.
And here's a quote: "“Baptism is a holy ordinance preparatory to the reception of the Holy Ghost; it is the channel and key by which the Holy Ghost will be administered. The Gift of the Holy Ghost by the laying on of hands, cannot be received through the medium of any other principle than the principle of righteousness." History of the Church, 3:379; paragraph divisions altered; from a discourse given by Joseph Smith on June 27, 1839, in Commerce, Illinois; reported by Willard Richards.
Also this lesson from the seminary manuals invites teachers to draw a door on the board and then describe the symbolism of keys, one of which is the Key of Baptism. The key opens the door so we can live with our Heavenly Father again.