Your problem and mine is that the word "electricity" isn't well defined. To one person, electricity is the movement of electrons in a wire, to another person, electricity is the movement of positive charged forces in a wire. A better term for "electricity" would be "the transfer of power or energy", but even adults sometimes have difficulty wrapping their heads around that concept.
Foss Science Resources focuses on the electron as to what is physically moving; the school district's answer key focuses on the traditional "movement of positive forces". The distinction between the terms is better taught in high school physics, but you're stuck trying to explain it to your fourth-grade students.
For me, rather than saying "electricity flows" one direction or another in a wire, I say "electrons flow" in a wire. I talk about electrons flowing because in a wire, electrons are the only things that actually move. The individual positive charges on the atoms stay fixed-in-place, but as a group, the positive charges appear to move.
I'm not sure this will work for fourth grade students, but you might talk about the Foss Science Resources explanation as the direction that electrons move.
Something you'll have to work around is the politics of not using the school district's answer key. Most of your students, however, will never really deal with the electricity direction conflict. The few students who will be dealing with the conflict will have a better base of understanding if they are taught the scientific "electrons flowing" rather than the ambiguous "electricity flows".