The solution to this problem is for you to establish clear definitions of what your children can and cannot expect from Mom after school. Begin by limiting the number of school-related questions you will answer after two o'clock to two per child (maybe three for the younger one). Tell them that after they've finished as much of their homework as they can (and not before), each of them can bring you two (or three) questions, but that you'll spend no more than a total of ten minutes per child answering them. After the ten minutes, they're on their own.
I guarantee that if you enforce this dispassionately, your kids will (a) begin to pay better attention during school hours and (b) eventually all but stop asking after-school questions. And you'll be able to get your Mom-work done. Independence in a parent-child relationship is always a mutual thing.
Family psychologist John Rosemond answers parents' questions at parentguru.com.