How to convince my parents let me keep another kitten?

We already have 5 cats in the house but one is going up to the barn so there will only be 4... that sounds like a lot but we have a very large 3 story victorian home so it only feels like we have 2. Anyway, my grandparents have two horses at a horse farm near their house, and they have tiny kittens there that they are trying to give away. I fell in love with a little black kitten but cant convince my mom or dad to let me bring her home... seems ive tried everything... any suggestions on how to convince them?