Yep, if you can't get them to eat a balanced diet... Same applies to meat, doesn't it? You might as well say "if you can't get them to eat meat"...
I hear this BS all the time... "Being vegan is dangerous if you don't get all your essential nutrients". Fact is, ANY diet is dangerous if you don't get all your essential nutrients. And the standard Western diet is one of the most likely NOT to provide all your essential nutrients.
Most vegans are much more aware of dietary requirements than most average people. So perhaps you would be better directing your advice elsewhere