tl;dr
Take the vegan out of this and we see a family entering a cafe/bar with food that they've bought elsewhere. This is rude and a lot of eateries would ask you to leave for doing that and most of us would agree that the staff were quite within their rights to do so.
If the child will only eat McDonald's, then, I don't know, maybe the whole family could eat there. Or maybe they could get the child something to eat there, make sure it's eaten before going to the cafe/restaurant. Because it's rude to bring your own food into a cafe/restaurant and owners and staff have the right not to serve you.
Bringing the vegan back into it, it's a bit bloody rude to bring meat into a meat and dairy free environment. Would it bother me? A bit. Probably to the same degree that it bothered @TapasForTwo. Enough to notice it, enough to think it was rude and enough to maybe post about it on here. And enough to think it was hilarious that some people seem to think that all vegans are mentalists who want to take all their meat away from them forever.
And yes, I don't eat meat. I ate meat on and off for most of my life then about five years ago I read a novel, We Were All Completely Beside Ourselves and suddenly I just couldn't do it anymore. N.b. the book only has some mild bit about meat can be bad in it, so I don't know why it hit me in the stomach so hard, but life is a funny old thing. This year I'm working toward a plant-based diet, sort of vegan, but I have lots of leather shoes and stuff, so much as I'd like to go the whole way, I know I won't. Blah, blah, blah. I hold my hands up and say I really don't like to see people eating meat around me, but that is my issue and not theirs. I happily judge on the inside while also happily judging myself for being such a hypocritical twat. I don't preach, it's not my place and everyone - including me - hates that shit. A friend did inspire me, but not through preaching, but because I was so impressed with the way he'd changed his life. I do, however, sometimes force tasty as fuck vegan dishes on friends who love dairy and eggs and all that good stuff. They cry and ask me where are the eggs, why is there no meat choice, you are destroying my life. And I laugh and tell them my victory is close. Yes, my tongue is in my cheek. Oh, no meat! 