Is it illegal to not update your income when dealing with child support?

In certain circumstances, yes, substantial changes in income must be reported.  I would need to know more about the particulars of your situation.  In my work as a practicing family law attorney, I consider people's finances when assisting them.  I consider your finances in setting legal fees and charges.  I also often assist people in a limited manner: This is technically known as providing 'limited scope legal services', and another term is 'unbundled legal services'.  I understand that finances are often difficult and so I will help people in a limited manner, in order to provide you with quality legal advice and counsel for your situation.  In my legal practice, it is not uncommon to assist people in completing legal documents and forms that pertain to Minnesota legal issues.   In my view, yes, I recommend you have attorney assistance.  You raised matters of concern for which an astute attorney can privately advise you to make requests of the deciding court official (a judge, a referee, a child support magistrate) in order better to protect you and the children from this point forward.  When choosing an attorney to help you I recommend making several phone calls because it is so very important that you feel a sense of great safety and trust in the attorney you choose to help you.