I don't know what cash is good for. Any question with "saving for x with y vs cash" the answer is always Y. Saving cash is the stupidest thing. You can buy stocks, you can buy gold, you can buy bonds, you can buy real estate if you are rich enough
However you save for your kid, do not save with CASH, cash devalues, save with something instead of cash. Which brings me to the topic, I would rather save for my kid with bitcoin than cash for sure but I would diversify and get a lot more than just bitcoins.