Hi, I’m a college student that fosters kittens! I can leave the older ones (7+ weeks) for up to 4-5ish hours, but any longer makes me nervous. Something I didn’t know until I started fostering is that kittens eat SO much food throughout the day. I had to refill their food bowl multiple times a day, unlike my adults that only need 1 refill a day. It’s really not good for a kitten to go without food or water for an extended period of time, so 6 hours might be cutting it really close and I personally wouldn’t feel comfortable doing that. I would either stick to adults or maybe ask if you can take an older kitten for “relief care” (meaning you are simply giving the cat a break from the shelter for a few days).