Led by founder-conductor Charles Ansbacher since 2001, the Boston Landmarks Orchestra has put on free classical music concerts each summer at myriad venues throughout Boston and its surrounding communities, as well as in the cities of Cambridge and Quincy. While the orchestra’s programs routinely feature the works of populist powerhouses like Beethoven and Gershwin, perhaps the ensemble’s most distinctive contribution to the classical music repertory is its annual commission of a new composition specifically for children. The resulting works for orchestra and narrator are performed as part of the Landmark Concerts for Children series.