Nonsense, of course it is. I'm raising bilingual children and every one of them has developmental delays. Of course the plural of anecdote is not data, but as a result I've been quite motivated to keep up with the research. My family is not exceptional among bilingual families. Developmental delays are common, as are poor performance in elementary school as multi-lingual children struggle to integrate disjoint systems of knowledge.

Of course I wouldn't be doing it if I didn't think it was absolutely worth it, but the idea the children are sponges that pick up language without trouble at all is a myth that needs to die. Children do learn languages faster and better than adults, but IMHO that is mostly due to (a) time, (b) motivation, and (c) adult learners rationalizing their failures rather than actually trying to learn.

EDIT: But to be clear, although it is not my place to tell anybody what to do, I do wish speakers of dying languages would pass on their words and culture to their children. It's hard, but worthwhile IMHO.