Why did Peter's mom make Fudge lamb chops but not him in Tales of a Fourth Grade Nothing?

Peter thinks it is mean when their mother makes Fudge lamb chops because he isn’t eating and she thinks he’ll eat if she makes him his favorite foods.

When Fudge refuses to eat, the family goes to great lengths to accommodate him.  They even let him eat under the table like a dog.  Fudge’s grandmother tries to trick him into drinking milkshakes by telling him there is a surprise at the bottom.  He drinks the first one and then throws the glass in anger when there is no surprise.

Fudge’s mother even tells Peter to stand on his head to amuse his brother.  Peter learned to stand on his head in gym class, and is proud of it.  Doing it at mealtimes is another story though.

Finally my mother got the brilliant idea of me standing on my head while she fed Fudge. I wasn't very excited about standing on my head in the kitchen. The floor is awfully hard in there. But my mother begged me. She said, "It's very important for Fudge to eat. Please help us, Peter." (Ch. 3) 

It doesn’t work.  Their mother decides to take Fudge to the doctor, but the first one says he will eventually eat.  That was not what she wanted to hear.  Peter says he could have told her that for free.

I guess my mother didn't believe either one of us because she took Fudge to see three more doctors. None of them could find a thing wrong with my brother. One doctor even suggested that my mother cook Fudge his favorite foods. (Ch. 3)

Peter thinks this is mean. The rest of the family has to eat stew, and Fudge was served lamb chops!  He doesn’t eat those either, though.  He says he wants Corn Flakes.  Peter’s mother gives him the chops and gives Fudge cereal.  He doesn’t eat that either, and his father dumps it on his head.  After that, he eats.

Why didn't Fudge eat the lamb chops in Tales of a Fourth Grade Nothing?

To put it mildly, Fudge is a stubborn child and a very difficult little brother for Peter. To answer this question, we need to look at the events that lead up to Fudge's mother serving him lamb chops for dinner. The incident begins when Fudge decides to stop eating, right after Peter learns to do handstands.

By the third day, their mother is concerned by Fudge's refusal to eat, and the family attempts to coerce Fudge to eat in various ways. His father does tricks to entertain him, but nothing works. Only when Peter stands on his head in the kitchen does their mother succeed in getting some baked potato into Fudge's mouth.

Over the next week or so, the family indulges Fudge's ridiculous requests to get him to eat. After that, however, Fudge is taken to the doctor, who tells his mother that Fudge will eat when he gets hungry. This answer wasn't good enough for Fudge's worried mother, so Fudge winds up being taken to other doctors, one of whom suggests that their mother cook Fudge the foods he loves best.

This leads us to the incident with the lamb chops, which are Fudge's very favorite. They were prepared to perfection, and Peter was very jealous that he was expected to eat stew while Fudge got to have lamb chops. However, Fudge's stubbornness and desire for attention prevails, and this is why he refuses to eat the lamb chops.

Throughout this novel, Fudge shows himself to be a selfish, objectionable, and attention-seeking child. The reason he didn't eat his lamb chops was because he wanted to see how whether he could continue to be difficult and how much of his nonsense his parents would take from him.