Bilingual even trilingual

When you are going to the park with your kid, you sure meet there all kind of other kids.
That's exactly what happened yesterday to me, when I and Bryce were playing in the sand and a bunch of kids were building sand castles in close proximity. One of them, a really cute, curly boy starts speaking with me in a perfect English, asking me if I'm Russian. I'm just amazed. How can such a small kid distinguish my accent? I asked, in return, why does he think I speak Russian. His answer: "Because I speak it". At that point, I'm more thinking to myself, yeap, kids like to invent stories, blah, blah. Soon enough, his father comes:"Nathan". The boy turns around and starts speaking Russian fluently.
I wanted to spank my inner me. How couldn't I trust the little one? And in the same time, keep talking Romanian, French and any other language, momma, 'cause these little ones know their business. :)