Can we prove it with documntation? No. To my knowledge there is no established line of descent that demonstrates living descendants of the Egyptian pharoahs. Can we safely assume that they exist? Yes, and we can also assume that, if they do, a large percentage of the world's population, if not all of it, is included in their number. I wrote an answer some time back that dealt with descent from figures ~1000 years ago, pointing out that all of Europe can safly claim descent from Charlemagne. The last pharaohs of Egypt lived 800 years before he did (assuming we count Ptolemaic), while the beginning of the line goes 4000 years prior, which is something like 200+ generations. A quick check shows that 2^200 = 1.60693804e60, and a little more digging shows estimates on common ancestry to be that "all modern individuals have identical ancestors by about 3,000 BC." So not only do the Pharoahs have living descendants, but you and I, and quite possibly every living person on the planet, are them.