It's called 'The Beginning of Life', and was released earlier this year. You should be able to find it on Netflix (will need subtitles for some bits). 

It's a documentary based on a lot of recent studies on the development of babies and children; how they learn, how very early experiences affects them, how their brains develop by play, discipline, and how affection really does play a huge part, even from birth. It also addresses how frustrating it can be, and how children are literally built to test their parents. Some of this might sound obvious to some of you, but it really did change my mind about a few things and hammer home a some truths. 

I'm planning on watching it again with my partner (preferably without bawling my eyes out at this one part that wasn't sad, but holy shit, emotions!) it's a really great documentary, and I hope some of you enjoy it too :)