Child support is typically based on gross monthly income. Any material change in circumstances could warrant a modification in child support; however, you should speak with an attorney to get an idea of what the modification might entail.
This is NOT legal advice, is GENERAL INFORMATION ONLY, and does NOT establish an Attorney/Client Relationship. Therefore my answer cannot address your specific legal situation and you should not rely upon my answer in your legal matter. I am an attorney licensed in Maryland.
Please note that the fact that you have another child with additional expenses for that child cannot be the only reason to reduce child support. Please consult an attorney. Also, consult an attorney to determine whether your reduction in gross income is a "material " change in circumstance.