Since your babies are pets, I would definitely do a care sheet. You should emphasize what they are used to as far as food, water (bottle or bowl) and other aspects. If you are working in litter training, mention that as well as what kind of litter. For food, write down the brand and formula so they know what it is as well as providing a sample of it. If they have had any fresh food, be specific on what kinds and amounts. You basically want general care for rabbits if the people don't know much, but also the specific care the babies have recieved so they have a starting point and can keep things the same if needed. It can also be a reference point, if the babies are used to one kind of litter, but the new owners use something else, then that could be a reason why the babies won't use the litter box.
Some local contact numbers are a good idea. Find some rabbit vets in the area and put down the info (phone number, website, address), if there is one you like, say that as well. If there are some good rabbit clubs, put down that info too. Have your info as well. Other care resources like websites or this forum as also good to include.