That may be practical, but many kids I observe in my family etc like to collect the money and see it and are proud of their collection and about what they saved. That goes away with a card ... and I wonder how that impacts the "feeling" for it. Counting and making likes and plans about what to buy is a big part of learning to deal with it.
Like or not the world is a digital currency world for the most part these days. I want my kids to understand that those numbers on a computer screen have real world value. How many young adults get into trouble with their first credit card or debit card because the money isn't "real" to them? In the US it's quite a few.
I don't have a final answer to this. However in m observation with different families and kids is that some start relatively early, before the kid can do arithmetics or such, they can count till a pile got enough money for the sweets or toy they want and then count the other pile to see how much is left and if they can buy the other.
Doing this in a digital system requires first some computing device (be it phone or laptop or whatever) then check the account value, read it correctly, then do some arithmetics and then interpret the result. That's a year or two older.
