it depend on what kind of lettuce.
do not feed them with iceburg lettuce.
first it has no nutritional value to the rabbit and second it is too watery.
it will cause digestional upset to your bunnies due to their sensitive digestive tracks.
other lettuce such as romaine lettuce are ok.
as for cabbage, some say ok and some say not because they are allergic to it.

as for me, i try to stay on the safe side which is not to feed them.
lettuce contain lactucarium which can give your bunny diarhea.
although some lettuce contain small amount of it, i still prefer not to take the risk.

their major diet rely on hay.
if you wan to give fresh vege and fruits for them, you need to be very careful with it.
you can refer to the first page for what are safe for them
monitor them carefully. any changes their poops or behaviour indicate that somethings is not right.