So the child was generalizing along the wrong dimension, so you decided the solution was to train an increase in generalization of the word meow which is what you got. You need to teach discrimination; not generalization. A method for doing so is to present the pink cat and pink fish sequentially. Reward the meow response in the presence of the cat, and reward fish responses to the fish. Eventually meow responses to the fish should extinguish.
So the child was generalizing along the wrong dimension, so you decided the solution was to train an increase in generalization of the word meow which is what you got. You need to teach discrimination; not generalization. A method for doing so is to present the pink cat and pink fish sequentially. Reward the meow response in the presence of the cat, and reward fish responses to the fish. Eventually meow responses to the fish should extinguish.