Bella is most likely just trying to play with your older cat Baby. Baby is probably bothered by this so it looks like they're fighting. Some kittens will play very rough. You may need to reintroduce them again very slowly.

Is Baby an active cat or does she like to sleep a lot and enjoy her peace and quiet? I ask this because if she doesn't like to play then this will most likely be an ongoing problem. There's also a very big age difference between the cats so if you can you may want to think about adopting another kitten or a cat that's more playful and closer in age to Bella. Good luck.