I have checked the 1901 census and the middle initial for John does not look the same as the middle intial for Charlotte and James and it certianly looks like a G to me. But you have to remember that the renumerator only goes on what he hears and he could have heard it as a G. The fact the John is the father, Charlotte is the mother and there is a James J is very telling and that they are living in Newport Pagnell. Almost certianly the correct family.
MMN is mothers maiden name. On the birth registers after September Quarter 1911, the mother maiden name is logged.
Jo