If you're making a knitted toy for a baby, you probably don't want to use safety eyes. But embroidering eyes on a knitted surface with yarn often doesn't look as good as you would like. I rarely embroider eyes because I dislike how they look, and it takes too many tries to get them to even look half-decent. I prefer to use felt.
You can just use a simple circle of black felt, maybe with a white dot added, stitched on around the edge. You could even add eyelids or eyebrows with felt or crochet. For a cat, you can use a coloured oval with a pupil of black felt stitched on. Or if you like embroidery, you can embroider eyes onto felt, and then stitch the eye onto the knitted face. I particularly like this last method.