TL;DR It takes slightly longer.
You might want to check out http://psychology.wikia.com/wiki/Toddler. It has useful information, such as the facts that 10-20 words can be taught. We all know that a child's brain development hasn't grown, yet it hinders their education, right?
Languages like Japanese for example have more grammatical aspects to it. It has polite and standard forms, and tenses based on animate and inanimate objects. Remember we're talking about someone's mother tongue! Children have absolutely no concepts of grammar when first learning it, they can't say "私 (watashi)" means I in English.
So, it's very obvious. Yes, you didn't want knowledge of things to do with second languages but here's one interesting thing. Now, the FSI (Foreign Service Institute) has Japanese as Category V (it means it takes about 2200 hours I think when I last checked), opposed to languages without rather complex grammar ideas.
There is likely going to be a psychological study, of course. However, it's common sense and checking out school curriculums are quite good.