>Any language will suffice.

Yes, a child could even learn lojban given enough time, but the goal here is to 
make a better language. In the case a of babies, they need to clearly 
communicate "My teeth are killing me, get me to a pediatrician" before they can 
say that in lojban or English.

Some children, including one of my sisters when confronted with a difficult 
language will stop speaking entirely until they are competent. My sister didn't 
speak at all until late and began speaking in complete sentences.

I think the "bioprogram theory" of creoles is applicable here. The default 
language that kids would speak if they had no guidance would be a highly 
regular *deep* creole with analytic syntax. I'm emphasizing *deep* because 
real creoles exist on a continuum of almost-like-English to essentially a foreign 
language. That deep creole is the one with the simplified grammar.

The textbook advice that "any language will suffice" is meant to combat racism 
and ethnocentrism that makes ill advised policy makers think that the 
barbarians and natives will be civilized if only the kids will stop speaking Inuit 
and speak English instead.

Matthew Martin