What about using the Bank of Dad? Like in
http://www.mrmoneymustache.com/2015/05/20/what-im-teaching-my-son-about-money/Since the kids are young and the numbers are small, just keep track of their balances and pay them interest. No need to worry about actual Vanguard accounts or account minimums until the numbers become large enough to warrant such things (or they have actual earnings to put into a Roth IRA). IMO the important lessons while they are young are that saving money now translates into more money later, and they have to be able to afford all the things they might want to buy with the amount of money they actually have. You can even highlight these facts when you pay them the interest, like "wow, having your money invested paid you the same as picking up all those leaves, but without any of the work!"
Eventually you will want to convert their fake bank accounts into actual investments (maybe in their teens?), at which point you can teach them about owning actual businesses like Coca Cola, and why your investments may go up & down in value.
I don't have kids though, so take that into consideration :)