Salaam brother! My father always used to tell us that little babies can see angels and I heard the same thing about mentally disabled people (those incapable of going through the test of this dunya). I recently dealt with such mentally ill children and looking at their behaviour ("talking" to the air, laughing at random) I can see where the idea is coming from, but is there any truth to that in Islam, or is that just some folklore-kind of thing?
Alaikumassalam wa rahmatullah,
The Quran mentions a few rare instances where humans see angels, as in the case of Mary, mother of Jesus (peace be upon them), who speaks to an angel who appears to her like a man. There is also the story of Abraham and the angels who visit him and his wife, giving him and his wife the news that they are about to have a son (Isaac). But those are rare, miraculous instances that do not necessarily have any bearing on our day-to-day lives. For more about the interaction between the Seen and the Unseen world please see my essay Al-Ghazali’s Matrix and the Divine Template.
As for babies having special powers to see angels, there is no evidence in Islam for that (archived link to a fatwa on this), it seems to be a Middle Eastern superstition.