When Marie suspects that the fish has been stolen by the cat, she forgets that the cupboard door was closed when she went to look for it.
The telegram from Louise, forgiving Marie, is dated 15 September 1914. Germany invaded Belgium on 4 August. The long voyage to America in addition to the plot complications would have probably taken a lot longer to resolve.
In Pollyanna (192)), Pickford could get away playing a 10 or 11 year old.
In Through the Back Door, one year later (1921), she cannot pull it off: she is a year older (29 vs. 28), her chest is not flat, and her face is too "womanly".