Colleges that also look at the CSS (where you can explain these kinds of things in detail) are really trying to allow for individual circumstances. Most schools I’m aware of start discounting when you have just two kids in college simultaneously, so I would certainly think you’d get some consideration from most of them.
Call the financial aid office of any school your youngest is applying to and tell them of your family situation – ask if there’s any way for you to highlight this information in the financial aid folder. Then notify the colleges of your older kids too.
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