Bitcoin can not replace the dollar and will never be a tool for exchange of goods and services, can you imagine when you buy pizza using bitcoin, when bitcoin prices go up twice or ten times whether you will pay the same bitcoin? Bitcoin prices can change dramatically in just minutes and hours
A kid knows about dollars because America is a dominant country. Most people know America. We all think that the lives of Americans are all better than ours. There's even a term called the "American dream". But that's all besides the point, my point is that America is a dominant country, right? So, what is the currency that they are using? Dollars. That's why most people know about dollars. I'd like to disagree that dollar is an international currency at all since it's not accepted everywhere. There are countries that are friendly with major foreign currencies but most countries aren't. You'd have to convert your dollars first to their local currency before you can spend it. Just like how you'd spend bitcoin in that place.