Easing the transition:
Find adopters for them now, while they are young. Have those people visit for a while until the kits are about 12 weeks old; that way the kittens are used to their new owners at least a little. And you get the benefit of getting to know the people who will adopt the little ones.
Another benefit of waiting until 12-ish weeks: You can spay/neuter the kittens; get a vet who does pediatric spay/neuter. That way your adoption fee can pay for the spay/neuter, and the new owner doesn't have to worry about it, nor be annoyed that s/he is paying a fee to adopt non-purebred kittens (or, even worse, worrying that you're trying to make money off the kittens, which of course you're not).
Send their old food and litter with them; also a towel or blanket that's been in their cage, for familiar smells.