The Little Pair indeed burned their diaries during the captivity at Tobolsk, however the volumes of 1912,1913 and 1916 (of GD Maria Nichoaleivna's diaries) survived. I hope my memory still serves me well.
This is correct. Nicholas mentions in his diary on 9/22 April 1918:
"We learned that the extraordinary commissar Yakovlev had arrived from Moscow...The children imagined that he would come today to perform a search, and burned all their letters, and Marie and Anastasia even burned their diaries."
As you can see, Nicholas doesn't say precisely how many diaries the younger girls disposed of. IMO, the Little Pair certainly burned their most recent diaries -- presumably 1918 and 1917. The surviving volumes that belonged to Maria may have been packed away in crates in the storage sheds, like her father's. (In Ekaterinburg, Nicholas's diaries were stored in a crate labeled "NA 13.")
