I just don't understand those that are saying you should force her to eat what you cook. I would never force my kids to eat something they don't like, I would hate for them to end up with some food phobia!! I've always had the policy that I will ensure they are getting sufficient protein, fibre, vegetables, dairy etc but I'll give them what they like in those food groups.
To the person that said she won't be getting any b12, that is not true if she eats fish, dairy products, eggs etc. Vegans don't get any b12. Vegetarians should be able to meet the requirement without eating meat.
I do agree a multivitamin would be good though as means you don't have to worry so much about all vitamins etc.
Honestly, she sounds like she eats ok for a 6 year old. As long as she's eating eggs, dairy, fruit and veg, some fish, she's doing a lot better than some kids!!
I would probably have a conversation with her to explain that if she's going to be a vegetarian, you'll support her but she needs to try other foods. You say she used to like meat, so maybe explore veggie options like quorn, tofu. Yes it might be processed, but no worse than sliced ham etc that some kids eat a lot of. And it's a good way of adding more options into her diet.
My 16 year old daughter decided she wanted to be a veggie 6 months ago and she's soooo fussy so I feel you're pain. Deciding what to do for dinner every day is difficult. But I try and batch cook stuff for her so I can pop something in the oven... bean chillis, lentil bolognese (blended so it just looks like tomato sauce!). She's just tried tofu in a restaurant for the first time. She eats a lot of cheese and eggs. I just do the best I can with what she likes and encourage her to try new things xx