It is unclear what you are referring to when you say “Separation Agreement”. MA does not recognize legal separations;it does however allow for actions for Separate Support. Agreements for separate support address only the custody and financial needs of the (custodial) spouse and the children (they do not address distribution of marital assets). Unlike in other states, such as New York, where a separation agreement can convert into a divorce action, MA requires the commencement of an entirely new action if the couple decides they want to terminate the marriage after a period of separation.
If however,you are referring to a Settlement Agreement prepared to address all the elements and issues in the dissolution of the marriage, and subsequently incorporated into the Judgment of divorce: these Agreement are always open to modification in regards to any issues involving the care and custody of the children, even if the Agreement does not merge with the Judgment.
and seek to increase that amount to meet the child support guidelines. This however may be mitigated by other “trade-offs” you may have conceded as part of the negotiations for the final Agreement.
Modification cases are highly particularized and require an experienced attorney to navigate. If you have not done so already, you should consult with an attorney to review the Agreement and Judgment and help explain your situation in light of the facts of your case.