I think that earnit is more of a joke. Piggybanks always had an appeal to be phisical items where you store small amounjts of money like change. Why create such a thing for electronic transactions? The fees involved and the required trust in the third party that has to come in to store them e-money completely negates the concept of a piggybank for me.
The behavior of small children are changing due to the mass adoption of tablets and smart phones, now even 3 year old is playing around with a tablet all the time. They grow up in a digital world so it is natural to use digital money, just like in any game
A bitcoin hardwallet should be the right equipment for this kind of piggybank, it works exactly like a piggybank, your bitcoin only exists in this hardwallet. But if many children start to use it, the transaction capacity should be a concern, so only large saving is practical due to fee (But it is easier to understand fee instead of inflation)
Following the same thought, you can have a piggybank for pensioners, where they buy 1 bitcoin each month for their retirement saving account, and the return of that piggybank will be 16% per year guaranteed. With a good fund manager, the return can even reach 30% per year, so that early retirement is achievable at ease. The fund should be accessed by multi-sig to reduce the risk