Except that children can walk with assistance (and crawl) before they're strong enough to walk on their own.

Interestingly they can also effectively communicate well before they can vocalise words. A baby can sign reliably at 7-9 months but typically won't be able to speak until 12 months.

Yes! Signing (albeit with a small vocabulary) was surprisingly successful with my infant daughter, over 20 years ago. I was initially skeptical, but it worked really well. Not sure if it reduced my infant daughter's communication frustrations, but it certainly helped her parents feel less frustrated.

'Walking by one, talking by two'

But if you hold a child up before their legs are strong enough they will walk.

Or maybe it's not the brain is to small, but that the brain is still growing and therefore subject to enough noise to make actions now impossible for the sake of the future.