Q. Is it permitted to count a katan (6 or 7 years old) for a minyan if he is holding a sefer Torah? Is this a fulfillment of tefilla b’tzibbur? Or should I leave and go daven in a normal minyan with 10 or more adults?

A. Shulchan Aruch (O.H. 55: 4) rules stringently not to use a minor, however, Rema (ibid.) mentions that some are lenient in need. However, Mishna Berura (ibid. 24) quotes from many Achronim not to count a minor even when holding on to a sefer and in time of need.
Some Poskim are lenient in need or when there is concern that the minyan will close down, and under certain additional conditions. (Maharsham 3: 162, Igrois Moishe 2: 18, Chelkas Yoab 1: 93, Betzel Hachochma 4: 3, et. al.).
Horav Shlomo Miller’s Shlit’a is to follow the Mishna Berura and be stringent in all cases.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a