I don't personally think that they are, as human infants don't have the ability to break them into bits and there can be screws and things on the inside. Also, just because something is safe for a human chemically, doesn't mean the same hold true for a bird (think about avocado, chocolate, perfumes etc). Anyway, in one of Max (the cockatoo's) videos on Youtube, he was playing with a bunch of baby blocks. His owner was shocked when he discovered that Maxwell was carrying around a small metal screw. It came from one of the blocks, so he took the rest away.
That having been said, I have a feeling a lot of people still give these to their birds.