(1) I’m with you on the toy hamster point. (A pet hamster in a sealed tank will be a cruel and bad idea for a question. A cockroach instead perhaps?).
(2) CO2 dissolves in water slowly (stale water is more acidic). If that’s a good datalogger then it will show [CO2]↓. O2 won’t dissolve as much in one day and it won’t show on datalogger.
(3) Depending on the food, bacteria and fungi might break it down to a certain degree in 24 hours, via aerobic respiration in this case most likely. Looks like the answer chose not to consider this factor.
To a P6 kid, I might stress on point (1) and point (2), and put very little weight on point (3).