If I were you, I'd set them up an account with the bank of Dad/Mom. You can do all kinds of things, like give them high enough guaranteed rates of return that they can actually notice, 2x the S&P500 return on the upside only, nice stuff like that. As they get older it can get closer to, and become, the real thing, but for now it's nice to just get them excited about investing.
They can help with the accounting part too, rather than having a bank tell you what your balance is they would have to do the work with you. Very educational.