Hello Kaz. I find it very encouraging that you've got older vegan children (as lots of other people do, obviously) because I keep getting 'Wait till he gets to school, he'll want to eat what all the other kids are eating and will go behind your back", which I find not particularly helpful. My son is 19 months and very healthy, so I think people like to make problems where there are none, because surely being vegan can't be that easy can it?
Where in the UK are you?