I'd recommend looking into a Roth IRA, its more flexible than a 529 and what you don't spend can go towards your retirement.
But with a Roth IRA, it must be in her or her husbands name and they can only withdraw the contributions penalty free, which doesn't help if they want it to grow more for their children. Although it does solve the problem of saving for the children vs. saving for retirement.
If you just wanted a normal investment account, Betterment (
www.betterment.com) might be an easy way to go. You could open up 1 account and create goals for each of the kids. Then you can easily adjust the allocation for each kid separately based on how many years until they need the money. You can make deposits for as little as $10 and be diversified across up to 12 ETFs. However, when you have less than $10,000 in your account, you'll need to automatically contribute $100 a month or be charged a $3 per month fee. As long as you do $100 a month, the fee is 0.35% of assets on accounts with less than $10k, 0.25% of assets on accounts over $10k but less than $100k, and 0.15% of assets once your balance is over $100k. Keep in mind that with a taxable investment account in your name, you would be the one paying taxes on dividends and capital gains. So at the very least, you'd probably want to keep enough to cover taxes once you finally withdrew to give to your children.
Another possible thing to do with the above money would be to contribute to a Roth IRA in your children's name once they are old enough to start working and earn an income. So if they earned $2,000 with a part time job, you withdraw $2,000 from this investment account and put it in a Roth IRA.
With a Betterment account, you could also create an IRA "goal" for yourself in the same account. So if you are able to, you could contribute to each of the kids' savings while also contributing to a Roth or Traditional IRA.
Otherwise I don't know much about 529 plans, but I believe that if they were to withdraw money from a 529 plan and NOT use it for education, they would have to pay income taxes plus a 10% penalty.
