I've hesitated on crocheting cat toys due to safety concerns. Claws may get caught in the threads of the yarn and yarn can be eaten and cause blockages. If you do crochet toys for them, I'd consider using yarn thats made of one solid thread rather than traditional yarn, like the velvet blanket stuff. I've seen small balls of thinner velvet yarn at dollar general before that might be perfect for it.

Also, don't give them the toys unattended, and always remember to throw them away the second they start unraveling or breaking. I worked at quite a few vets offices as an intern and a tech and saw many people come in with animals with intestinal blockages who were shocked because they never saw their pet eat anything odd. Hair ties, string, and parts of toys are big ones for cats to eat. Catnip inside may encourage them to chew or eat the yarn.

I know it sucks because you want to make cute toys for them as a way to show love, but unfortunately cats can be dummies. We still love them anyways!