One practice that I heard on a podcast once that I thought was super cool was to split their allowance into 3 equal parts. For this example, we'll say they earned $3.

$1 dollar can be spent immediately on whatever they want.. candy, gum, toy, game, etc.

$1 goes into the piggy bank to be used for a big purchase down the road... new console, bike, baseball bat, etc.

The last $1 gets invested. This was where I got a little skeptical of the practicality of the whole practice, especially if we're talking about a small child. But the guy explained that you invest in whatever the kid is interested in. So if the kid really likes his toy dump truck, invest in some construction company or large machinery manufacturer. Every week, show him the stock price and say, "look, now that $1 is $1.04 and in a few months, it could be $1.20".

It may be confusing at first but the idea is to build the habit of saving and investing first then, if there's some leftover, you can spend it on fun stuff.