They feel the stress of it/lose control of their posture

(At 1:53 video) They feel the stress of it, here I don't understand to what does "it" imply, but I will try to guess, does it refer to the actual situation the baby is in?

Lose control of their posture- Posture is in other words their body?

And then, we asked the mother to not respond to the baby. The baby very quickly picks up on this, and then she uses all of her abilities to try and get the mother back. She smiles at the mother. She points because she's used to the mother looking where she points. Dah! The baby puts both hands up in front of her and says, "What's happening here?" She makes that screechy sound at the mother, like, "Come on! "Why aren't we doing this?" Even in these two minutes when they don't get the normal reaction, they react with negative emotions, they turn away, they feel the stress of it. They actually lose control of their posture because of the stress that they are experiencing.