How can a child "appear to have rickets"? Either they do or they don't!
A well balanced 'vegan with added fish' diet is about as good as it gets nutritionally, provided the parents are not relying on processed meat and cheese substitutes which are even worse than the real McCoy.
Social Services are barking up the wrong tree here. Rickets is on the rise not because of hippy vegans, but from kids spending most of their time indoors, paranoid parents slapping on factor 50 whenever the sun comes out, and an increase in the number of parents who cover their female children up from head to toe.