YABU - if there wasn't going to be a choice, you should have checked what the kids could and couldn't eat. As others have said, this isn't just about veggies, but also about other dietary requirements (not fussiness - I haven't got much time for that - but genuine requirements, such as only being able to eat Halal meat).
As a veggie myself, the rule of thumb seems to be that, if an invitation to a party asks whether I'm veggie or not, it's up to me to say that I am. But if the invitation doesn't mention dietary requirements, then it's fair for me to assume that there will be a choice.
I imagine this little girls' mother thought the same thing - who would imagine that, at a kids' party, the only food option is going to be crappy processed pork or nothing? If it was 1985, yes, but vegetarianism is hardly uncommon these days.
However, among my friends and family, about 50 per cent of us are veggie, so I accept that I may have a different view of things than others who have grown up thinking that eating meat is the "norm."
As it would have been just as easy for you to provide something that everyone could eat, such as cheese and tomato pizzas, or just have half cheese sandwiches and half hot dogs, then I would say YABU. Especially as you accept you had three days to check with the kids as to what they could and couldn't eat.