This form is rarely needed. You only need to do a Multiple Support Declaration if you are claiming someone other than a qualifying child as a dependent and there are two or more people including yourself who provide support for the dependent.
If you have a regular dependent (qualifying child) such as your son or daughter under age 19, you don't need to file the Form 2120. The definition of a qualifying child is a child who is your son, daughter, stepchild, foster child, brother, sister, stepbrother, or a descendant of any of them (such as a grandchild, niece, or nephew) who was under age 19 at the end of 2019, under age 24 and a student, or any age and permanently and totally disabled, who didn't provide over half of his or her own support for 2019.