1. It would appear that the actions of the mother are not conducive to fostering the development of skills in Mary. At the age of three, most children are messy eaters, and often require a change of clothing an; they also frequently require toileting assistance. Further, by not allowing Mary to eat with the other children, the mother is impeding the development of social skills. Keep in mind that overly protective parents feel that they are doing the right thing, and are generally oblivious to the fact that their actions are a barrier to the development of independent skills.
2. The mother should be approached in a non-confrontational manner. Ask the mother for her opinion as to Mary's developmental progress, and make tactful suggestions for changing the routine. If the mother feels that the teacher is "teaming up" with her to assist Mary, she may be more receptive to suggested changes. Offer to confer with her about Mary's progress. Ask Mom to observe Mary at home, and report her observations to you (the information she conveys may be of little use, but persons are more receptive to an idea if they feel a sense of ownership in the process). Do not infer that the mother is the root of the problem; instead, solicit her help in assisting Mary.
3. I would not ask Mary's mother to leave. If she is intent upon continuing, she will do so regardless of the teacher's requests to the contrary. The most important reason, however, is that the mother will most likely become hostile and defensive, destroying any chance of gaining her cooperation. It may take time, but there is a chance that Mary's mother can be persuaded to try a change.
I hope this is helpful.
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