No, you do get to pick which country, within limits. For example, the booth between Norway and Mexico, you have to pick between those two countries. There are other booths scatter throughout WS.
When we went in Jan., my DD9 and DS6 had a blast doing this. Out of 7 countries, they did, um, 7! And they have different missions for each country, so you could do the same country twice and have different clues.
A 6yo is likely to need help, though--my DS6 couldn't have done the whole thing without his sister. But, they both had a fabulous time. Don't be surprised if one mission is not enough!
P.S. I didn't actually do the missions with them--I found a place to sit, where I could keep on eye on things, but give them a touch of freedom. I wouldn't have done this with just one 6yo. I asked my two "experts", DD9 preferred China (maybe biased since she loves China and pandas), DS6 liked Japan. And Norway. Basically, they liked all of them. Here's another tip, though--the Mexico one is inside, so you're in air-conditioned comfort...