You'd want to make the caretakers as impersonal as possible, maybe even to the point of being robotic. Consider how much of communication is nonverbal. We even have languages that are completely based on hand gestures. Even the slightest reinforcement of certain nonverbal actions by the caretakers might have a profound effect on how language develops. Then again, if you had a thousand islands full of babies, you'd have room to systematically alter variables like level of contact between babies and caretakers and see what differences those alterations made.
Yeah, that's a good point! Deaf kids of hearing parents develop pretty advanced homesigns which might be a result of the building up of communicative skills gained via nonverbal communication. Susan Goldin-Meadow has a case study of a kid whose homesigns even exhibit recursion (although it's still heavily debated since sign data are notoriously hard to analyze). But nonverbal communication is probably a necessary precursor for language...hmm... Revised plan: 2,000 islands! Half with no nonverbal communication, half with haha.