I have a little one that just came through the screaming stage. Her parents think it`s funny so it`s hard for her to understand that I DON`T think it`s cute or funny. I just kept saying NO screaming, gave her my evil eye and would not feed her while she screamed. She also was not allowed down while she was screaming. It will come to an end...someday, lol. For the older ones I would take away their lunch and put them in a time out. You also have to remember not to use the chair as a babysitter. I know it`s hard with a busy little one, but if they are in the chair a lot, while you are cleaning up or making lunch or going to the washroom or doing something with the older children, the little one will think of the chair as prison! Also, does she have something to play with or distract her while sitting in the chair? How about playing some music during lunch time. Also make sure you say, good girl for not screaming today and give big hugs or high fives for the older kids, even a treat as a reward for not screaming.
Good luck