This thread is hilarious- veggie children ones always are! Lots of posters say - no one is saying you should give up meat, but perhaps you should give up meat.
Posters saying - you don't have to make 2 meals each night, just make a completely separate curry/pasta sauce...
And for the love of tasty food, if you are only going to throw the chicken or lamb in the curry at the last second, dont bother, you need it cooking in to get the flavours.
Anyway, I'd go with sitting everyone down as a family and saying a) they arent dictating how the whole family eats and b) its not reasonable to double your work load by expecting a separate meal each night.
So by Friday each week, you'll do a meal plan, there will be one night each week with an easy adaption you'll do, like sausage and mash or burgers and chips, and as it is no extra work to put the veggie versions under the grill, you'll do that. Otherwise they can decide if they want to eat what you are having, or part of it (like the veggies and potatoes from a roast, with a mushroom parcel instead of the meat), or if they are going to make something separate. They have until Saturday lunchtime to decide what that something separate is and you'll buy the ingredients.
You expect them to decide between themselves who will cook, who will clean up, and if they write a meal plan that's junk like instant noodles, this ends.
If they are grown up enough to want to go veggie for moral reasons, they are grown up enough to understand they must eat a healthy balanced diet.
If they do decide they want to eat what you are having, that's fine but they have to leave enough for you and dh/tell you early so you make enough for them as well. (No "oh actually I fancy a proper burger after all" when you've only bought 2!)