Maybe it’s something for them to suck on. Chain keeps it close, as babies are known to drop and forget things. Guard keeps it from going all the way in the mouth and choking the baby. And then made to look like a weapon because the nobility thought it was cool.

Or maybe it was a small knife intended to train them to handle a blade from a young age and be more natural at using it and if the baby dies from it, well that’s just what most babies do.