Hi, Darcie:I am sure that this is also deeply upsetting to the mother, as well. It's usually so embarrassing to see our children behave badly. I remember when my own children were small and I was teaching in the preschool where they attended, that my oldest son told me that he didn't like for me to be the teacher, he just wanted me to be the mom. The children could be struggling with having to share their mother. Do the tantrums typically occur when they see her? If so, then you may have to adjust the schedule or the environment a bit so that they don't have interaction with her. It might also be helpful to have the older child to take ownership of his behavior by talking with him at a time when he is not acting out. He is old enough to start having conversations about how his behavior affects the other people around him.I am sure that the example you gave at lunch time was just one example and that there are others. Usually with children who are acting out, I would do some type of reward system using a behavior chart. It is a concrete way for the child and his teacher to see his goals and to work toward an activity that is intrinsically rewarding. For example, if the goal is "Remain seated during lunch time," he could earn a star or sticker at each lunch that he accomplishes the goal. When he has earned 5 stars/stickers, he can have 10 minutes at the end of the day on Friday to play with a preferred toy. You have to be careful with this, though, because it should be a private thing and you wouldn't want to create tension with the other children. You also don't want his behavior to become dependent on rewards. Encouraging him to take pride in his own accomplishments would be the goal, eventually, but rewards are so valuable for young children because they are concrete.
I hope that these ideas help and wish you good luck with your situation.