However, unless Mom and sick child have APs, they will actually be losing a day from their tickets thru attrition. For the sake of argument, let's suppose that they family has 6-day tickets and pre-booked FP+ for all 6 days for Mom, Dad, Bud and Princess. Bud wakes up sick on day #3 and Mom stays back at the resort with him. Dad and Princess go to a park for the day. Mom and Bud now have one additional day of park admission on their tickets (which will expire 14 days after their first use) and the family is heading back to Heartland, USA the morning of day #7. Whether they used their ticket at the gate or allowed the unused days to expire, Disney still got their money for the full 6 days. Is it so unreasonable to at least allow Dad and Princess to use the FP+ that were reserved with Mom's and Bud's ticket? IMO, chances are very slim that Mom or Dad is going to go into
MDE and cancel those FPs so that someone else can snag them. Mom is too busy with a sick child and Dad is busy having a fun day with Princess. The FPs will simply go to waste.
Just playing devil's advocate here and throwing some food out there for thought.