Re: Criminal offence to impose a diet lacking in essential elements ?
I'd say that's not quite as bad. It's negligence rather than imposing poor health through ideology.
Actively trying to give your child an inadequate diet is more serious.
I suspect that the straw parents you complain about would be more likely to be trying to do the opposite. I am not sure that to fail in that goal (of providing a diet that is both vegan and adequately balanced for a child) is morally worse than a failure either to realise that a junk food diet can be harmful or simply not to act on that realisation.