You know how very small children chew on stuff on and off and then they outgrow the habit or the impulse disappears from their lives or whatever? You know how some older kids and adults never really stop chewing on things, even after they've freaked everyone out by sucking on their hair, chewing pencils to slivers (that make their gums bleed), etc.? There might be an anxiety disorder involved. Do everyone a favor (the person, their parents or other family members, people who are in their general environment, their teachers, etc. -- but mostly the person) and buy them some of these. You can get them on breakaway lanyards, so if the kid wears them around the neck and gets on a bike you don't have to develop an ulcer worrying about the strangulation hazard. If they tend to take off the lanyard and leave it places, you can get it on a retractable belt thing (like a badge or keycard at work) so they have it at the ready but it doesn't fall to the floor, into the toilet, etc.
The product claims to be BPA and pthalate free, which is good. Do _not_ believe anyone who tells you these are indestructible. My son has already put a hole through one of the train charms and he chewed on it for less than a week. But it only takes him twenty minutes or less to destroy a pacifier and maybe an hour to ruin a shirt collar. Days of chomping before disposal is a minor miracle.
Best of all, it truly does seem to satisfy and calm some basic and otherwise impossible to suppress urge.
Sensory University makes a bunch of other cool products (including the tangle, which I find very satisfying when I'm feeling an impulse to twist or fold or shred things). You can buy them a variety of places online, including Amazon. One of these products is something that fits onto the top of a pencil, nice for inveterate pencil chewers and even more stealth than the chewy charms (but, alas, chokable, so use some discretion with these around younger children).