Q. Dear Rabbi, In our small weekday minyan we have a problem getting someone to blow the Shofar daily. Can a youngster who is not bar mitzvah blow for us?
A. Nitey Gavriel (Rosh Hashono 4: 8) writes that if no other Baal Tokea is available to blow the Shofar during Elul, even a minor who is not Bar Mitzah yet, can be asked. However on the onset, a Koton should not blow the Shofar as it is not befitting for the honor of the congregation.
Mekadesh Yisroel (Yomim Noroim p.44) depends this Shaileh on the reason for blowing Shofar during Elul, if it is for awakening the listeners to do Teshuvo and a minor may not be as stirring and as effective as an adult. Other reasons mentioned apply even when a minor blows. (See also Meagel Hashana p.37 and Shevet Hakehossi 1:185).
Horav Shlomo Miller's Shlit"a opinion is that he can blow when no adult is available.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit"a