how can you tell the face and base apart ? on a triangular pyramid for a 4th grader

It would depend on the type of pyramid.

If it was equilateral (that is, all edges were the same), it wouldn't matter - something like identifying the base of a cube. (Trivia - this would also be called a tetrahedron)

If it was isoceles - that is, the edges of faces were the same - then I would take the base to be the one that had all three edges the same length, and the faces would be the ones with only two equal sides.