> “A child who is learning two languages will have a smaller vocabulary in each than a child who is only learning one; there are only so many hours in the day, and you’re either hearing English or Spanish,” Dr. Hoff said.

That is rubbish. It depends on how the child is exposed to the various languages and how much exposure they get. My daughter speaks 3 languages at home (one to each adult). We were told exactly this that she would be behind language wise, but we made sure that she was read stories in each of the languages, and had good exposure. She has just started at a bilingual school, and her monolingual English teacher has told us that she is far ahead of the rest of her class language wise.

Yes, this may be an outlier, but every family that I know that has bilingual or more children and who expose the children to enough of each language have children that are ahead of the norm in each of those languages.

If however you simply split the time that you would give in language related activities or conversation between multiple languages, then of course the child will be behind in each one, as you've only exposed them to half the language in each. This is true up to about 7 years old, and non-existent after that anyway.