The problem with feral children is that they don't interact with other human beings until they are past the so-called "critical period" for language acquisition. The primary purpose of language is communication, and if you don't have anyone to communicate with you're not going to start spontaneously producing language. And then by the time these children are exposed to language, it's too late for them to be able to fully learn it. Feral children are few and far between, but we have a lot of cases of deaf children whose parents don't know sign language, and then they don't learn language until they are much older. Studies show that they can't acquire native-like competence in sign language. Here are some papers on functional and structural brain abnormalities of late learners: 1 2. Here are some papers describing linguistic behavioral deficits of late sign learners: 1 2

Putting a bunch of babies together doesn't have this problem, though. If you're surrounded by other human beings then there are opportunities to communicate, and thus it's possible that language could emerge. It also circumvents the problems of late first-language learners of sign language: deaf babies are surrounded by adults, or children who already know a language, in their formative years. There's evidence that adults are much less innovative than children when it comes to learning and creating language, so we wouldn't expect for a new language to emerge between a deaf child and his/her parents.

In short, putting a bunch of babies on an island would be awesome because they don't have another first language to interrupt creation of a new language, they aren't socially isolated so they have motivation to communicate with others, and they haven't reached the "critical period" for learning language yet.