Can a vegan diet damage your child's health? Social workers in Lewisham believe it can, which is why they tried to take a five-year-old who appeared to have rickets into care. The boy's parents have just won their legal battle to prevent this, and they have also succeeded in having their son removed from the at-risk register.

The couple say they don't eat dairy produce because asthma runs in the family – but they're not vegans, as social workers claimed, because they do eat fish. However, the case raises questions about how difficult it is to nourish a young child adequately on a restrictive diet – and whether the risks involved are too great.

Paediatric dietician Helen Wilcock, a member of the British Dietetic Association, says she tries not to be judgmental about the rights and wrongs of vegan diets for young children, but any parent wanting to raise their child as a vegan needs to be very well-informed. "Vegan children can be deficient in vitamin D, calcium, iron and possibly vitamin B12, so they need supplements," she says.

Another big issue is that a vegan diet isn't very energy-dense: you have to eat a lot of it to get enough energy. But children typically don't eat a lot, so getting enough calories into them can be difficult. "I recommend adding oil to their food," Wilcock says, "because that gives them more calories."

Another difficulty is protein. "If a child eats meat and fish, it's easy to get all the right amino acids. But if a child is getting protein from pulses, the problem is that one type of bean might not provide every amino acid, so there has to be a good balance of pulses. In other words, a child who only eats chicken will get all the amino acids – but a child who only eats one type of bean won't."

So information is the key – but do families really try to raise their children on vegan diets without being adequately informed? Sometimes, says Wilcock, they do – often because they are taken in by misleading information on the internet. And when a vegan diet starts to go wrong, the first symptom is usually that the child fails to thrive or grow properly. It's the shortage of calories and protein that kicks in first, she says, with rickets (caused by deficiencies in vitamin D and calcium) usually much further down the line. "Families are then referred to a dietician like me for advice – and every parent I've seen has been happy to make the changes I've recommended, because first and foremost they want their child to be healthy."

The most challenging time for parents raising vegan children is when they are under five – although another crucial time is for girls around puberty, when iron levels can dip.

But the risks of inadvertently malnourishing a child aren't restricted to veganism. According to Claire Williamson of the British Nutrition Foundation, one of the mistakes parents can make is to assume, wrongly, that what's healthy for an adult is healthy for a child. "For example, semi-skimmed milk, low-fat foods and high-fibre foods may be best for adults, but under-fives need full-fat dairy produce, while high-fibre roughage can fill them up too quickly, so they don't eat enough nutritious food."

Roughage bulks out the diet and moves food through the gut, but it's far more important for adults than for children – which is why in the past, says Williamson, children on vegetarian diets were sometimes referred to as "muesli belt kids", because their diet was too high in roughage, leading to deficiencies in some nutrients.

Amanda Baker at the Vegan Society says the real issue isn't whether a child's diet is vegan or not, or restricted or not – the important thing is whether it's healthy. "There are plenty of children who are eating a bad diet, and they're not vegan," she says. "Vegan parents have to plan their child's food carefully. Of course there are pitfalls, but there are pitfalls for all parents and for any diet.

"The reality is that vegan parents are more likely to cook at home, and are likely to be very knowledgeable about nutrition because they have had to make a lot of effort to follow the diet they do. Many of them follow a wholefood diet, and avoid trans-fats and too much salt. It's actually much easier for vegans and their children to meet the five-a-day guidelines than for other people."

Vegans, she says, are victims of the fact that many people, from doctors and health workers to social workers and other parents, are badly informed. "We've written to every GP's surgery in an attempt to make sure there's better information out there. Parents can come in for mistaken pressure from people with genuine concerns, simply because the issues aren't properly understood."

Are your children thriving on a vegan diet?