Enter the current annual tuition and what you think your average tuition inflation and investment return will be. Then put in the years until your children go to school and how much you now have to invest and how much you will contribute each year.
Note that you can put away a year in each child's name in an "Coverdell Education IRA" where all the earnings are not taxed when you withdraw the funds. Similarly, many states now sponsor 529 plan savings programs. See www.collegesavings.org for more information.