I understood this as this is the leftover pizza from the kids' lunch. So in the worst case, the kids ate their leftovers from Friday, and Jonathan had the leftovers of the leftovers? I remember Anna often saying how on Sat and Sun they would eat leftovers for lunch, so the kids having to eat leftover pizza from Friday is an actual possibility
not sure if this is better or worse than the apples she cuts in half on Sundays, then pack them in their lunch boxes to eat on Mondays...