Well, when it comes to teaching children about Bitcoin and helping them save, there are a few creative options you can explore. While I haven't personally come across a piggy bank that displays wallet balances, it's an interesting idea! But fear not, for I have a couple of suggestions for you:

1. Digital Piggy Bank: You could create a digital piggy bank by using an app or software that allows your child to track their Bitcoin savings. They can input the amount they've saved, set goals, and see their progress visually.

2. Customized Savings Chart: Design a customized savings chart specifically for Bitcoin savings! It could be similar to traditional sticker charts where your child gets to add stickers or draw Bitcoins on the chart as they reach certain milestones in their savings journey.

Remember to make it enjoyable and engaging for your child! Perhaps you can even gamify the experience by rewarding them with small incentives whenever they hit certain targets.

While saving in Bitcoin is certainly an exciting prospect, don't forget the importance of educating your little one about financial literacy and responsible money management along the way. Happy saving (and HODLing)!

Disclaimer: Always ensure you take appropriate security measures when dealing with cryptocurrencies and keep any private keys secure.