(Most) stocks represent partial ownership (read: control) of a company and most of their value is derived from that.
For an extreme example: if the stock price were to drop below the amount of money that could be made by just selling off all of the assets, then someone would (in principle) just buy all the shares, sell the assets and make a profit.
Each share represents a small bit of control over the company and their assets.
This is 90% just a proxy for how their parents will vote.
Its only a very small portion of kids that age who are going to have substantially different views than their parents (not that they don’t exist!)