I was five when I became a vegetarian. My elder sister was seven. The thing is, she is old enough to have an opinion, I found that quite sad that you said that tbh and a little worrying because she may take that as a sign that she can't come to you with her opinions, so please be sure to reassure her on that, kids hold onto these things sometimes (or she may have forgotten it! I had a particularly emotional chat with a 14yo last night who remembered some specific things said to him from early childhood!)
As for mind changing, so she might change her mind, but she might not, I haven't eaten meat since then (over 16 years ago!) and gave up fish at 7, then gelatine and other additives at 11. My sister eats fish but hasn't eaten meat since she was 7 either.
As Esta said, it isn't a massive hooha and not that difficult. I've been cooking two separate meals at dinner for years now and they're always the same thing but one with quorn/soya/lentils/etc and one with meat. It's bloody easy! If you don't want to do two all the time, get your DH and DS to eat quorn sometimes, won't hurt, my boyfriend eats an almost exclusively vegetarian diet since moving in with me (we live with vegans who stated no meat tbf) and he's fine! He actually thinks quorn isn't too different. Prefers Sainsbury's soya mince to quorn mince though, not sure why. You don't have to use quorn/linda macarty, there are loads of inventive ways to use lentils and other pulses instead and they taste just as good as meat apparently!
Also, protein is not that hard to get either. You should be less worried about protein and more B12 and the things less commonly spoken about. Everything she needs can be found in a good, varied vegetarian diet. Just like how meat eaters don't get everything they need if they don't have a varied diet.
Tardis: I've been making boston beans recently veggie style and they can be done well, I'm still experimenting with the bacon aspect though.
OP: If you choose to let her become vegetarian, educate yourself and your daughter on what she actually needs in her diet, how you can get that in and inventive ways to do it.
My Mum didn't really do this and hoped it'd be okay. I became quite weird with food and it's only been the past few years where I've started recovering from that and am able to eat a good varied diet. I emphasise educating your daughter on her diet as she needs to know, and though she may not understand it too well, she will develop her understanding of it.
Some things that are great:
Chickpeas. In any form. You can make fantastic burgers with them, eat them cold and raw, eat dried chickpeas (middle eastern shops do them I think, not the type you soak!), warm them to have on the side of meals, use in bulgar wheat or quinoa salads.
Mushrooms are apparently 'meaty' according to my DP. We make lasagne's and bolognaises with finely chopped mushroom and sainsbos mince (half and half) and obviously whatever else is in the fridge at the time. I have a thing about mushrooms but am slowly getting over it and chucking them in a lot of other meals like curries and casseroles at the moment too!
Beans of any kind. Some taste great cold out of the tin (I think it's flageot i like cold with balsamic vinegar, the little green ones...), some warmed through, in salads, with main meals, etc. You can make a great butterbean roast easily enough and they're good to have with roast dinners imo, maybe not to everyones tastes though!
Dried fruit and freeze dried fruit. Freeze dried can be an acquired taste, but both are good for snacking on veggie or not. I live on dried and freeze dried strawberries atm.
Squash is great, any kind of squash not just butternut or pumpkin. Squashis really versatile, can be thrown into lots of things and my Mum makes a squash, sweet potato, red onion and garlic roasted mix when we have roast dinners at her house, it's divine! Can be had plain, or with pasta, mashed (that's good too, with a dash of orange juice especially!), baked, etc.
Greens can be experimented with easily enough. Worst comes to worse and she doesn't like them, spinach can be finely sliced/chopped/blended and added to food.
DP says potato and beans have been great. Very flashy (wtf?!) and filling. We do use both a lot though.
Lentils are good, you can buy cans, use in shepards pies, casseroles, etc.
Fruit smoothies are brilliant desserts or snacks and fun for everyone with picking out fruits and taking turns pressing the blend button, etc.
Also, 'hiding' veg in food, whilst a well known trick, isn't used enough. I 'hide' vegetables in a lot of my food, partly because of food issues I have (mostly over those now being honest), but also because it does add an extra boost of nutrients depending on what you use. Adding parsnips or swede to mashed potato. Finely chopped carrot/courgette/mushroom to mince based meals. Beetroot I hear is good blended and put into things. I have heard of people whizzing up veg in the blender into a paste and freezing it in ice cube trays to add to meals.
Jacket potato and beans is a good one, easy to whip up and accidentally vegetarian.
We use the Quorn Kitchen cookbook a lot. I have loads of veggie cookbooks but that has seen a lot of use recently! Mainly for ideas, as a lot of the meals don't need quorn in them really, but there are tasty recipes galore in there! I have a couple of student cookbooks too which are veggie and we use those a lot too, along with The Thrifty Cookbook by Cate someone. That's not a veggie cookbook but it has broadened both of our ideas a lot and now we consult that when faced with an empty fridge!
Also, I use less of the '5-a-day' philosophy in my cooking and more the 'eat a rainbow' one, much more interesting and lots of different nutrients across the colour spectrum in food!
Look for accidentally vegetarian meals, as opposed to thinking "Shit, I now have to cook something vegetarian!". And also look for not-so-mainstream things, like sea vegetables, which are brilliant on the nutrient side of things! You can pick up nori everywhere these days and flake it over meals. Nutritional yeast flakes add a cheesey taste to meals according to my vegan housemates and are full of nutrients. Nuts and seeds are full of good stuff, can be ground and put in food, eaten whole, seeds can be crushed and sprinkle over cereals and pastas, etc.
I apologise for the essay. I've been learning a lot about food recently from my housemates!