Bird beaks are sort of like fingernails, they will slough off from time to time as they grow and so long as it isn't excessive and you don't see any other signs of illness, it's usually fine. It can also be caused by lots of things like vitamin deficiencies, infections, etc. If you're really worried, you can take them to the avian vet for a checkup, I've done that and never had them turn up a thing. Keep an eye on them, note changes in behavior, feed a high-quality diet and you're probably not having a problem. I don't see anything, just from the pictures, that would strike me as unusual.