@gocubs, Great job on saving for college! +1Million others advice on 529 Plan and one that is Illinois based so you do get the state income benefit up to whatever the limit may be for IL. Max out the annual tax benefit only each year (you can contribute every year of college even if you just pay it right back out to tuition as well typically).
Then find the best CD rate you can for the rest... Marcus by Goldman Sachs and Ally Bank are pretty aggressive (Marcus is tempting for this case with maybe a 1 year and a 6 month CD to try to catch rising rates through 2019?) Then drop money into the 529s above as CDs mature each year to rake in that tax benefit. It's taxable income, but lowest risk for a sure expense coming soon. Call it $1K taxable on the CDs per year and several $100s on the 529 tax free?
In my state, tax rate is ~5% of income and it's $4K per kid per year that gets a tax break for it. So that's $400 right away for $8K each year on both kids.