Q. See question above. Can a mother recite “Al Hamichya” on behalf of the boys in her group and for the cakes that they ate, if they are already Bar Mitzva

A. As mentioned, if the boys are already Bar Mitzva, they should recite the bracha for themselves. (Shulchan Aruch O.H. 186: 2), therefore they can recite but only for other women that blessing.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a