Okay, i don't think they ever actually told what each character's middle name was. What do you think was their middle names? here is what i think:

Alexander Paul Keaton

Mallory LeAnne Keaton

Jennifer Francine Keaton

Andrew Tye keaton

that's what I think anyway

Here is an actual full name:
Elyse Catherine Donnelly Keaton

Alexander Peter Keaton

Mallory Laura Keaton

Jennifer Leah Keaton

Steven Richard Keaton

Elyse Catherine Donnelly-Keaton

Andrew Stanley Keaton