Hello Doswald – what great looking bunnies those are! Thats so fun that you have so many bunnies together in your house – I’ve got 3 at the moment, but I can’t get all three to get along 
It sounds like Sadie is just a bit scared, but I wouldn’t stress yourself out too much because it does sound like Sadie is very happy if she is stretching out and relaxing in her cage.
A friend of mine works a lot with socializing animals (like cats, parrots, and rabbits) and she told me one of the best ways to get an animal to warm up to you is hand feeding. When you give the bunnies their veggies, try having your daughter hand feed Sadies – if she’s nervous about Sadie biting her, she doesn’t have to push the veggies at the bunny, but just hold them upt o the cage, even feed her through the side of the cage. Also, you could cut up a baby carrot into a few peices and hand feed those – this will help the bunny associate people with happy feelings!
Also, it could help to spend some time just next to the pen – reading, or talking to Sadie, or just hanging out – without trying to pick her up or pet her. This will help her get used to you, and help her become more social. Do this with the cage door open as well, so maybe Sadie will venture forward. Is there a pen you could put up for Sadie? You could set it up for and hour or two each day right outside her door, then open the door into the pen, and that way she could feel more comfortable getting out of her cage to hop around.
Her lunging and biting definitely sounds cage territorial – its quite frequent that bunnies will protect their cage like its their burrow, and is not them lashing out at you, just them trying to defend their home. When bunnies do this, it is best to get your hand on top of their head where they cannot bite you, and push it down gently, and pet their forehead. This will help them calm down.
I hope Sadie starts to get less nervous around people – patience is the best thing for a nervous bunny! My boyfriend has a bunny with a very similar personality to Sadies, and he spent a lot of time laying on the floor next to his bunnies cage – now, he naps next to the bunnies cage, and the bunny comes over and grooms him!