Check with one or more bike shops, especially ones that work on or sell kids' bikes frequently. That's not a dig on the bike, you're just more likely to find parts at those places if it's something specific to one-piece cranks or the department-store type bikes.

Other than that, you're probably out of luck. You could try to contact the store you bought it from to see if they can help....every once in awhile you get lucky that way. Or maybe they can clue you in to the distributor or company so you can contact them directly.

If none of that pans out, you could always buy a replacement crank or crank arm pretty cheaply if needed. Kinda hard to help you any better than this without knowing what kind of crank you have or what part is missing (or at least what the part is supposed to do), but I'm tryin'! )