When you fill out a W9, you are essentially saying that you are an independent contractor and responsible for your own taxes (as opposed to an employee, subject to payroll withholding.)
If you earn $600 or more from any one place where you have on file a W9, they are required to send you a Form 1099 at tax time (instead of a W-2)
It will affect your income taxes, because you must report it as income. Just like a W-2, a copy of a 1099 is also sent to the IRS, so you don't want to ignore it.
It will also affect your college financial aid, because that is based upon your tax return.
As far as child support, I am a bit less familiar with that, and I think it varies state to state in how they compute it. If all of your earnings are made this way, they would probably look at your schedule c from your tax return, and take into consideration business expenses as well.
If you make even a large percentage of your income as an independent, you also need to be paying your estimated taxes quarterly, as well as self-emplyment (medicare) taxes.
It would probably be a VERY good idea to talk to a tax professional for advice.
Hope this helped a bit.
Teresa