Learning Greek mythology in 4th grade is what set me on the path to Classic's grad school and teaching, great stuff!
riverboatking's suggestion of auditing a myth 101 course is a good idea, although you run the risk of them being incredibly boring. It might look silly, but a good place to start is
D'ulaires book of Greek Myths - it has nice big pictures, but is also a required text for UVa's intro mythology course. If you don't mind textbooks too much,
Powells "classical mythology" is decent and has a lot of primary sources.
El D's comment is also great, that area of mythology is fascinating. Joseph Campbell has written a lot of stuff about myth archetypes.
In terms of the essential primary sources the Iliad and Odyssey are of course must reads. Fagle's Odyssey is good as long as you're not a hard core snooty classicist, and for the Iliad
Lattimore's translation is the standard.
Ovid's Metamorphoses, although dense, is a great Latin version of all the myths. It all started with
Hesiod's Theogoney, but that can get a little crazy.