Celtic thunder, you didn't say how old your nail-biters are. Are they elementary school age? Pre-teen? Teenager?
Would something as simple as nail polish help (even a few coats of clear) simply because it makes the nail thicker and harder to chew? (Not to mention pretty, which might be a help as well.)
One article on nail-biting says this:
"Nail biting is the most common of the so-called "nervous habits," which include thumb sucking, nose picking, hair twisting or tugging, and tooth grinding, and is the most likely to continue into adulthood. About a third of grade-schoolers and half of adolescents bite their nails, and between a quarter and a third of college students admit to still gnawing on theirs.
"Nail biting is often a way of relieving the intense but transient tensions of childhood. All children get anxious. Learning something new in school or feeling shy at a party or on the playground are common triggers."
Here is the article, which includes some tips on what to do:
Here's another article, which points out that 60 percent of 10-year-olds bite their nails. It discusses such solutions as painting a bitter substance on the nails (which calls the child's attention consciously to the habit), a gold-star system, or having the child help you figure out how they can stop: