Money is merely a token for the exchange
of goods and services. Its "value"depends on the availability of goods and services and is negotiable according to shifts in that availability. Obviously, as the world's population increases the total amount of goods and services tends to increase, provided that the world's natural resources continue to underpin demand.
The way you teach children about money is to imagine a barter community in which the plumber is offered tomatoes from a grower in exchange for his work. The plumber doesn't like tomatoes but takes an IOU note from the grower for "ten pounds of tomatoes" which he takes to the baker and exchanges it for bread. The baker then has the option of claiming the tomatoes or exhanging the note with somebody else, and so on. That IOU note is
"money", whether it promises "tomatoes" ,"gold" or anything else considered "valuable".