Providing dandelion leaves for rabbits to nibble on is perfectly fine. They love these little flowers in fact. Each bunny is different and has different tastes however. Always start by introducing a little of a new food to see how they react to it.
I’ve been giving my rabbits dandelions for years. They also grow all over my garden and my buns nibble on them when they are roaming around. Which is a good reminder to make sure there aren’t any flowers or plants growing that you wouldn’t want them to find and nibble on.
If they aren’t eating the dandelions, or you see they are sick afterwards then stop giving it to them. Dandelions are flowers and need to be given in small amounts too. Flowers are more of a treat than a proper food.
Rabbits need a lot of good quality hay available to graze on to get their fibre intake. They also require some pellets and fresh veggies each day to round out their diet.
Give them some dandelions by all means, as long as they are eating all of the above. Nothing wrong with treating them a little bit. If you don’t have any growing near you, it’s easy to pick up some seeds and grow some.