Both Jeremy Jamesons and Joshs answers are good examples, each having their own advantages and disadvantages. We have a group activity, and a solo game. Allow me to offer a third choice which has its own different advantages and disadvantages, a 2 player competitive game.
The version I came up with uses Money, but you can use almost anything you have a lot of. Money just has several perks compared to most other times, such as the fact that they should know the inherent value and power of money.
How you play is simple, each person starts with a set amount, and each person gets a certain amount of money per turn. One gets a set number of money per turn, a linear amount, while the other gets a compound amount. The players then take turns getting their money until many rounds later when the compound is the clear winner.
A solid example (using American currency here):
You both start with 5 pennys(or one Nickel). The linear growth person gets 2 pennys per turn, while the Compound growth person gets 1 pennys per 5 they already have per turn (no fractions)(a more visually simply way to view that, one penny per Nickel).
Using those rules, the linear path starts off stronger, but after the 12th round, the compound pulls ahead by a penny, and the gap just gets wider from there.
Like this, if the child chose linear, they will likely feel the superior choice, easily beating the adult early on. This shows that the linear choice feels good early on, because in the short term, its better early on. After a while though, the Compound method catches up and then surpasses it, showing that its better in the long run.
There is an essays worth of things you can do or say to help the child understand why compound wins in the end, but that is far to long to really get into in the answer (maybe Ill throw some thoughts in the comments if needed/requested). There is also the issue that the linear player looses, and many children hate loosing, among other things.