All of my rabbits get fresh greens daily, and that includes does with kits. As long as they always get them, they never have any issues. The belief that fresh greens kill young rabbits stems from the fact that people buy a baby rabbit who has never had anything but pellets, bring it home, and give it lettuce/carrots/etc., and the rabbit overeats because they taste so-o-o good! :icecream:
Your kits are still young enough that they are still in the "tasting" stage, and getting most of their nourishment from nursing. I always feed hay first followed by greens. Does with young out of the nest and "growouts" get grain at the same time- dry does and bucks get grain only at night. I have 24 holes and six growout cages, so by the time I get back to the first cage fed, they have already been filling up on hay and grains- say 20 minutes or so, longer if I haven't harvested the fresh greens before feeding because I got a late start that morning.
Begin as you intend to continue- if fresh feed is going to make up a part of your herd's diet, feed the kits that way from the beginning. It is much easier than having to slowly transition older rabbits to fresh feeds.