interestingly, the learning curve difference in languages applies principally to the learning of the written language. Spoken languages all have pretty much the same level of complexity. Even "creoles" (spoken languages that develop in isolation) have comparable levels of complexity to any other spoken language.
Also interestingly (despite the fact that I've completely lost where I heard this from, so could be completely wrong), I remember something about Japanese high school students being unable to read more than a very basic set of kanji.
Yeah, young children are pretty much preprogrammed for language acquisition, and that preprogramming pretty much guarantees they'll learn the basics of whatever language they're exposed to (disregarding speech disorders, learning disabilities, and severe retardation or brain damage).
If you want to give your kids a fighting chance in the multilingual world of tomorrow, raise them multilingual, and get them really interested in reading and intellectual pursuits at a very young age.