Is it ok to let baby to fall down for them to learn? Will it hurt their brain? As I don't wish my kids too protective also. My baby 8months now. I bought a thick carpet for her (2inch). She always unaware and fall behind while sitting.

Babies are quite resilient but just make sure that it's not too often. If the fall is too hard, the baby cries inconsolably, has become listless, and has projectile vomiting, then this is not alright and head down to the ER. You may want to buy a head protector too. It's like a padded helmet without the hard outer shell. It cushions the baby's head on falls. I find it quite useful when my baby has started crawling and then when he was practicing walking.