Because ты is in a singular form but its use is in an informal way, on the order hand вы has a plural character (like you in english) but it also acts in a singular form in formal speaking. I would use вы despite of "you" in the following sentence, would you mind if I open the window? I won't use ты because it could sound a bit rude
It's ok if he's a child. Also it's ok if that's the only name you know him by. "Ваня" is usually short for "Иван" but it might turn out to be a shortening of some other name or even a full name on its own. Making assumption about one's full name when you don't know it for sure - that can come of as rude and put you in an awkward situation. Also in modern times it's entirely possible that you've known him over the Internet for a long time but only meeting him in person now.