Obviously they learn their native language quick because of full immersion plus playing and talking is pretty much the only thing they have to do. For other languages, if they spend say 3 hours a week learning another language they'd probably be at the same level as an adult spending the same amount of time. And things like curiosity, feeling of relevancy etc. are going to be a bigger factor in speed of learning I reckon.
Obviously they learn their native language quick because of full immersion plus playing and talking is pretty much the only thing they have to do. For other languages, if they spend say 3 hours a week learning another language they'd probably be at the same level as an adult spending the same amount of time. And things like curiosity, feeling of relevancy etc. are going to be a bigger factor in speed of learning I reckon.