What are the best Bitcoin "piggybank" options for children? I have three children age 4-7 and I want something where they have the ability to save in Bitcoin, see their stack grow, and then spend it as appropriate. Or maybe even borrow against it at some point. Having interoperability with the fiat system <I think> is still important so they can realize some sense of converting fiat into Bitcoin, and see how the value of Bitcoin goes up in fiat terms. I'd prefer a subaccount on my account so that I don't need to use separate logins for each one. Maybe @strike has something in the works?