I'm not sure which state you are in, but I know in Vic all parent volunteers need a working with children check. It is worrying though that a qualified aide is being replaced by a parent who, as experienced as she may be with children, may not have appropriate training with a special needs child. I would be concerned too incase the lack of adequate support results in disruption, or lack of attention, for the rest of the class. Perhaps you should raise this with the school principal before the year starts and see if they are able to reassure you?