The child support guidelines can be found here if you want to take a look at them: http://www.mass.gov/courts/childsupport/guidelines.pdf
The guidelines do not require an exchange of tax returns, but if you agreed to provide your tax returns in your divorce agreement, then you should follow the terms that you agreed to, or you could be in contempt of the agreement. As explained in the other answer, child support can be modified, but the courts generally discourage parents to come back to court if there is only a slight change in income. Generally, a 20% difference is considered to be a significant change that could result in a change in child support, BUT both parent's incomes, along with health insurance and daycare costs will also be considered. Your ex (or Mom) will also need to complete the required court financial statement, and attach a paystub and a W2 and/or 1099 for the previous year.
Although the child support guidelines may seem straightforward, the devil is in the details. For example, some people may not be working to capacity or perhaps have some unreported income. Using a divorce or family lawyer can help you with the details and alert you to issues you may not have considered, etc.
Hope that helps. My firm has additional info that you may find helpful on our website: www.divorcecollaborative.com
This answer should not be considered legal advice. Each case is fact specific, and not enough information is available in this forum to provide any lega advice to you, just very genera information. By answering this question, we are not entering into any lawyer-client relationship.
In most states that is standard language and yes you can request hers also. You can easily look up that law in your state or consult a local child support attorney.
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The exchange of returns would be mutual, and yes, each year you could be adjusting the amount of support. Child-related issues are always subject to being reopened upon a material change of circumstances. Since the guidelines formula is pretty cut and dry, you can avoid a hassle by coming to agreement on whatever the new amount of support your incoomes then dictate, and filing a joint modification.