Cash should be seen as medium of exchange rather than store of value. If you saved for a bike, the bike was probably worth much more to you than a picogram of gold. Even if you ended up forgetting about your penny jar and finding it in the attic as an adult, the act of saving itself gave your valuable lessons about planning and delayed gratification.
On the other hand, if you made even a conservative investment like a thin silver chain or a popular book with author autograph, I bet you will get more than your money's worth for that today. If you bought a share of Apple stock - woah, but I guess we couldn't have all known back then.
As adults, yeah we should know better than to keep our wealth essentially in the pockets of politicians. Invest in something that will be useful to your or someone else even many years later, or profit-generating businesses. But there is nothing wrong with children play-learning a simplified version of life rather than immediately having to know the difference between mutual funds and ETFs.