Big bro's philosophy on babies & knowledge

Since Lil Bro was born, Big Bro has held steady on his belief that babies are born knowing everything. I'm not sure where he developed this idea, it seems to have just arisen with him. He's apt to say that Lil Bro knew everything, but that he's slowly forgetting it. I asked 'at what point do babies start to forget'? According to Big Bro, it's almost immediately, as soon as they are no longer newborns (he defines a newborn as a baby with an umbilical cord, once the cord falls off, one is no longer a newborn and graduates to a 'baby').

This reminded me of discussions over Plato's beliefs during my college philosophy classes. I had to go back to look a few things up: 

Innatism is a philosophical doctrine that holds that the mind is born with ideas/knowledge, and that therefore the mind is not a 'blank slate' at birth, as early empiricists such as John Locke claimed. It asserts therefore that not all knowledge is obtained from experience and the senses.

In other words, there are some things that we just know, maybe by faith, maybe by instinct, maybe by the collective experience of humanity.

I'm fascinated that Big Bro plugged into this idea...