May one count a child over 9 for the tenth person in a minyan if people need to say kaddish?
There is a minority opinion which allows this, however from a halachic point of view this position is highly questionable and it’s difficult to rely on this on a practical level. Poskim state that those who rely on this leniency should only do so if otherwise the minyan will fall apart completely, i.e. there will not be a minyan on a regular basis. To do so on the occasion when there’s no minyan is not acceptable.
In practice, this should not be done at all even in those circumstances, and it’s better to daven biyechidus regularly than resorting to this leniency. The Rebbe specifically writes that this is not practiced among Chassidim.
Even those who permit joining a child for the minyan permit it only if the child is holding a Chumash in his hand, and according to many Poskim – a Chumash which can be rolled up, i.e. a sefer Torah. Even so, the Chazan should not daven the silent amidah first; he should wait till the minyan finishes the amidah and then recite his own amidah out loud. Only the following three kadeishim may be recited: after Yishtabach; before Ashrei and after Uva ltziyon.
ראה שו״ע או״ח נה, ד ובנו״כ. שו״ע אדה״ז נה, ה. אפיקי מגינים שם יז. שו״ת דודאי השדה ס. אג״מ או״ח ב, יח. אג״ק ח ע׳ רעו.