Elizabeth p. donner houghton’s recollections of the donner party’s tragic experience can be considered a memoir

Yes, because the book contains Elisabeth's recollections of things and events that she had been a part of, as a child. Back then, she was too young to understand the meaning of all that they'd been through, but nevertheless, her remembrances are striking. She also writes about her life after the "Donner Party" misfortunate adventure. That is the reason it can be considered a memoir - it talks about people and occurrences from a personal perspective and temporal distance.