I haven't waded through the whole thread so forgive me if this has been said already, but...
I think it quite possible that a veggie on ethical grounds is more passionate about their reasons for not eating meat/bringing their children up as veggies than those who just eat meat because they like it and its part of their culture. So, on that front I can see why your DH would be very unhappy if your children, especially the younger ones who will not understand the full concept and ins and outs of animal cruelty in this issue would be upset.
I confess that when my 13 yo step brother was encouraged to eat meat by my adult s/sister and permitted to by our dad despite the fact that s/bros mum would have gone absolutely fecking ballistic had she known, I was shocked and I didn't approve of the adult's actions - however, S/bro was old enough to chose himself, very intelligent and well up on the surrounding issues.
I'd also argue that you agreed that your DC would be brought up as veggies so I think that DH would be justified in being cross that you wanted to change the agreement on what he clearly considers an important moral matter.
But... but... as a Mum and one who is determined not to be railroaded over her choices concerning her own kids I can see why you are effectively saying, hang on a minute, they're my DC too, where's my say!
The problem is, to a veggie with moral reasons for his choice, there is no middle ground. It would be as wrong for him to say that the DC could eat meat half of the week as it would for him to agree to the idea wholesale.
I will ask you this - do you really feel as strongly that your younger DC, who aren't able to make an informed choice themselves, should eat meat as your DH feels they shouldn't? And if so, would that be because you feel you have a right to bring them up according to your culture and beliefs more than you feel that your choice is morally good and right and that the alternative is cruel/inhumane?
Now I'll admit where I am coming from. I'm a veggie of over 30 years and my DCs (12 and 14) have been veggie since birth, because of my ethical beliefs. I am pretty sure that I would fight far harder to keep my DC veggie than anyone would fight for the opposite choice, because I believe passionately, with all my heart, that to kill an animal unless it is suffering beyond help is wrong.
Hence I can see DHs point of view but boy I would be angry if as a Mum if I felt that I wanted to bring my DC up in a certain way and someone, DH or not, was opposing me and I feel your frustration.
Naturally though I side with your DH on this as I share his morals and can only say that maybe on this, if as I think may well be the case, his feelings are as strong and as passionate as mine are on this emotive issue, you may have to wave a white flag, only because you don't feel as strongly. Hopefully when and where there are other, just as important issues where you disagree but where passions are not so high on DHs side, you will have the ruling hand and the say in your DCs upbringing will be balanced out.