First off, we have to contend with her witless inability to remember that a small child is not a tiny grown-up who fakes acting like he or she doesn't know what to do just to be a jerk to Mommmy. Just as she stupidly expected five year old Mike to be grateful for her badly-done alien costume, Boomer Nitbrain Elly probably expected her children to pop out of the womb with perfect table manners and got frustrated and angry when they tried to trick her into thinking that maybe children have to taught things as part of a long-term process. Since she's too damned impatient and stupid, the former plastic flower child probably glommed onto the poor child's plastic cutlery and angrily demonstrated the right way.
Since her brain is simply two neurons tied together in a slip knot, it confused and angered her that her children behaved as if someone were trying to take their food away just because she lost her shit and tried taking their food away every so often. It also bothers her that guide books filled with lies and pictures of also-lies tell her that she has to set some sort of example. Not only does the angry dunderhead think she's perfect when she's not, she resents the idea of having to work towards something that should come instantly.
Finally, we have to remember that they have to wait hours on end to eat swill. It would be one thing to have to go hungry if the meal tasted good but Elly's cooking is a nausea- and indigestion-inducing mess that deserves nothing better than CHOMP-CHEW-GLUT-SHHHHLUUURRRP-SSSSSHHHHH
Simply put, it's hard to teach good table manners if you shoot yourself in the foot as often as Elly does.