It's not about vegans announcing they're vegan, it's about buckeejits being, well, buckeejits - & in that instance, taking the form of I am A VEGAN!!!eleventyone! I've met one of those. She was terribly upset about having to share the dairy-free margarine with me on a big Guide camp & I couldn't break into her monologue about her veganism to explain that there was no risk at all of my contaminating the tub with animal products, because I'd not be eating them either. She also refused to attend reflections because she was Buddhist. Reflections being a vaguely spiritual(ish) thing rather than religious (very literally a quiet time to reflect, with a reading or two designed to inspire, no prayers or anything). There are heaps of people who'll chunter on about their pet passion, working it into every conversation - it's really REALLY not "a vegan thing". Difference is, sometimes vegans actually do need to make people aware they're vegan - what with that whole business where eating food is a thing 
As for "how dare vegans force their children to be vegan!?" - surely meat-eaters are "forcing" their children to follow a particular diet just as much? In the UK, certainly, the "normal" diet isn't a healthy one - hence not only the obesity level but the fact people are suffering from malnutrition despite being overweight or obese: this 2009 article in British Journal of Nutrition noted that, on their single-day study at an acute hospital in North East England in 2007 Interestingly 9·5% (ten out of 105) of patients defined as at high risk of malnutrition with a MUST score of 2 or more were also obese with a BMI [of at least] 30 kg/m2. It seems reasonable to assume that number would now be higher were the study to be repeated. Obviously one can be vegan & overweight or obese (so please let's not try to sidetrack the thread into "ugh, smug skinny vegans") - my point is that the modern Western Diet is having a catastrophic effect on the population of the country; so "Oooh, being vegan is soooooo unhealthy! Won't anyone think of the children!" just sounds a bit daft. You can, of course, have a spectacularly unhealthy vegan diet - at either extreme of the plant-based spectrum, especially now there's so much more vegan[-friendly] junk-food (easily) available & so much stuff out there essentially promoting disordered eating. The immediate assumption that vegans are Doing It Wrong, though, is weird. And rather offensive, really.
As far as children developing rickets goes, if you read what the NHS had to say after the 2010 press coverage, there's no mention that veganism is a risk factor. Dietary insufficiency is mentioned, yes, but if vegans were the ones predominantly affected, you'd expect there to be a wee mention of that - it's the sort of thing God gave us brackets for... Obesity, however, gets a mention; as does the possibility of results being skewed by the ethnic diversity of the population (with caveat that places with less diverse populations have seen a similar increase in vitamin D deficiency); & of course [lack of] exposure to sunshine is the bit of the research that made the headlines.
I have had osteopenia so severe it was right on the borderline of osteoporosis. I've an anaphylactic allergy to all dairy products, but it wasn't being dairy-free that did it - my hips were barely osteopenic at all. Massive doses of oral steroids all through my teens & into my early twenties had basically strip-mined my spine; & I got to see someone headdesk IRL when my GP asked me what my last DXA results had been like & I told her I'd never had one... I'm only considered at risk of osteoporosis now because I've to take frequent courses of high-dose oral steroids - my diet's never caused concern, presumably because answering a couple of questions reassured my consultant that I know what I'm about.
It seems as if a lot of people haven't read the OP's posts where she explains that what her 11yo wants is to eat animal products when he's out of the house, in order to fit in with his friends. Which she's agreed to. So no question of him breaking out the mouthwash in the McDonald's loos & then coating himself head-to-toe in Superdrug/Sainsbury's-own version of Lynx before entering the house. Given he's just wanting to eat meat to fit in with his peer group, OP continuing to only provide plant-based meals at home is perfectly reasonable. Even, to be honest, if he were no longer wanting to be vegan, I don't think it would be unreasonable for OP to continue to only provide plant-based meals. You can't make a direct comparison to when a child/teen wishes to become vegetarian or vegan, because those are dietary changes where the existing household meals are no longer suitable, so they need to be provided with an alternative (in some form, which might mean cooking for themselves). A child/teen in a vegan or vegetarian household who wishes to add animal products into their diet should of course be free to do so - but they're still able to eat the household meals & lots of vegetarians & vegans are genuinely physically repulsed by meat/fish so cooking it would be utterly anathematic to them. So yes, the change from vegetarian/vegan is not simply the reverse of the change to it.
Oh & all the PP merrily piling on to call OP Worst Parent Ever & Abusive (etc) - did you miss the PP who's letting her DD with lactose intolerance just have whatever she fancies when they're out because soya milk's "rank" & "yuk" & she doesn't want to go without or endure said soya rankness. I don't think that's abusive, to be clear; nor do I think that's Worst Parent Ever material. I do, however, think it's hugely ill-advised & storing up all sorts of problems for the long-term. Her DD's lucky that there is now a lot of choice (relatively speaking) for people following dairy-free diets & those choices keep expanding. There are other milks to try at home, some of which are increasingly available in coffee shops etc. If it's eating out: allergen-labelling makes avoiding dairy when buying picnic-type-stuff very easy & there are quite a lot of Accidentally Vegan snack foods; restaurant-wise, lots of UK chains now offer vegan menus which ensure Safe Food for the dairy-free & in lots of cases make pudding (& sometimes pizza) possible where normally it wouldn't be.
It just baffles me that posters are so willing to accuse OP of half-killing her children (& of course causing irreparable psychological damage to the 11yo, who's also going to leave home asap - for a job in an abattoir, one assumes - & never return) when Metoodear openly admits to [facilitating the] doing [of] something that makes her daughter unwell. I totally understand it is well-intentioned & she doesn't give any idea of how often her daughter is having dairy etc; but while obviously the need to avoid it isn't comparable to the need for someone with an allergy to do so, why would you habituate your child to having something that makes them unwell? People of all ages cope with not being able to have all kinds of things; & if diagnosed in (early) childhood it's much easier to cope with Just Never Having Allergen/Cause-Of-Intolerance. Parents of Brownies I've had with food allergies &/or intolerances have made sure that they find suitable snacks for their DDs & if necessary provide them to ensure their child doesn't feel they're missing out. (Not necessary at Brownies, because anything we do with food's suitable for everyone in the Unit. Am meaning things like checking with birthday party hosts about food & then - most of the time - sending a packed lunch along to the party; & having dairy-free chocolate buttons ready for when their child's been given/missed out on sweets they can't eat.) I can't make a comparison with dairy versions of things, but there are definitely some dairy-free chocolates that are nicer than others & all of them are vastly improved from 10 years ago. The rate at which my Brownies demolish vegan s'mores & the fact they ask to make them every time we have a planning session rather suggests that dairy-free chocolate is acceptable to plenty of children who've no need of it. Well, nobody actually needs chocolate of any kind, but in the sense that they could have chocolate with milk in without any problems.
As for vegans all being angry & militant
The vegan population isn't some kind of amorphous plant-fuelled hivemind with a monomaniacal bent. Although the Vegan Society's definition of veganism* is clear that being vegan means, for, example, not wearing wool or leather & only using cruelty-free toiletries there are an increasing number of people who are "just" following a plant-based diet but refer to themselves as vegan; so you can't even confidently assume a shared set of values. That gang don't exactly tend to be militant. More weirdly evangelical about kale & avocado. And they are, if anything, almost suspiciously perky, frankly. Within the vegan population there is a significant crossover, as one might expect, with animal rights activists. People do tend to get quite angry-frustrated about the causes they're passionate about. Then you've the rest of the vegan population, who you'll find in all walks of life & all states of mind. Just like people who follow other diets. It's almost as if (wildly unhelpful & hugely frustrating) sweeping generalisations & tired stereotypes are nothing more than that...
- A philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of humans, animals and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals."