The cover of Vegan is Love is deceptively cheerful: There are smiling elephants, zebras, pandas, and even pink lambs. But inside Ruby Roth's new children's book, you'll find wounded animals. Rabbits trapped in laboratories. The slaughter house and the circus. Blood. "Killing an animal is not brave—it is cowardly. What we need today are people with the courage to protect animals, not hurt them," writes Roth, a former elementary school art teacher and self-appointed animal rights activist. "We can choose to live without using animals for food, clothing, or fun. As vegans, we live this way because it is best for our health, for animals, and for the earth … and that is love."

Vegan is Love (North Atlantic Books, $16.95) is designed to inspire children to adopt a vegan lifestyle at an early age. It's aimed at kids ages 6 and up, and includes lessons on animal cruelty and the environmental consequences of eating meat, such as pollution emitted by animal farms. Critics argue that it focuses too heavily on violence against animals. And some say it's unwise to graphically promote a restrictive vegan diet to young, impressionable readers.

The book arrives as vegan diets are making a media splash. Actress Alicia Silverstone, author of the vegan-centric The Kind Diet, is raising her 2-year-old son, Bear, as a strict vegan. And last year, an 11-month-old French baby on a vegan diet died after suffering complications from vitamin deficiencies; his parents were sentenced to five years in jail.

But that doesn't necessarily mean a vegan diet is dangerous, or even a bad idea. Indeed, "veganism can work with a child—but very carefully," says Katherine Tallmadge, a registered dietitian and author of Diet Simple: 192 Mental Tricks, Substitutions, Habits and Inspirations. "It's doable, but parents need to be very knowledgeable, since a child's nutrition needs are so intense."

While vegetarians eliminate meat, fish, and poultry, vegans go further, excluding all animal products—even dairy and eggs. That means saying goodbye to refried beans with lard, margarine made with whey, and anything with gelatin, which comes from animal bones and hooves. The vegan approach revolves around fruits, vegetables, leafy greens, whole grains, nuts, seeds, and legumes.

The reasons to go vegan are compelling: Research shows vegans tend to eat fewer calories and weigh less than their meat-eating counterparts. Plant-based diets are also thought to keep heart disease and diabetes at bay. "If it's done well, it's one of the best dietary patterns," says David Katz, founding director of the Yale-Griffin Prevention Research Center. "But all that glitters isn't gold. A marshmallow or jelly bean diet could count as a vegan diet. It is possible to do it badly."

The Academy of Nutrition and Dietetics supports well-planned vegan diets for infants and toddlers, but urges parents to pay special attention to vitamin B12, vitamin D, calcium, iron, protein, and fiber. Vitamin B12, for example, is found only in animal products. To get enough, vegans can opt for a supplement or fortified foods like soy or rice beverages, cereals, and meat substitutes. Getting too few calories and too little protein to support growth is also a concern, says registered dietitian Melinda Johnson, a lecturer in the nutrition program at Arizona State University. Meals ought to be built around a protein source like soy products, beans, nuts, or nut butter. And since vegans cannot have dairy, suitable replacements such as calcium-fortified soy milk or juice are necessary. "As long as these nutrients are replaced with the right plant foods, or a supplement, then it's safe for a child to go vegan," Johnson says. "If these nutrients aren't replaced, it can be quite dangerous." Since infants and young children require ample nutrients and minerals to grow and develop—their brains and bodies are transforming rapidly—most experts recommend consulting a pediatrician or registered dietitian before making the leap.

There's no set age when it's "OK" for a child to begin a vegan approach. Typically, mothers should breastfeed for four to six months, but once a baby begins solid foods, it's appropriate to replace meat products with mashed or pureed tofu or beans, along with soy yogurt and cheese, Johnson says. Still, parents and doctors should monitor growth to make sure vegan kids are getting sufficient calories and protein.