They aren't required to wear certain numbers. In fact, the jersey numbers are all over the place (there was a 00 in goal, for example, in a game I reffed this weekend....that was awesome but my favorite was the 69). Traditionally, however, there have been numbers assigned to certain players (see mirage's wikipedia article) though that has fallen out of fashion (with the exceptions of the 1, 9, 10, 8 and a few others which are still regarded as prestige number). US soccer assigns certain numbers to the various positions in the game (see the article I posted above)...the kids aren't required to wear those numbers on the jerseys...but the coaches do use the numbers in the planning method that US soccer presents for the licensing exams. So when you talk about the "1" that's the goalkeeper (whichever kid it might be) or the "10" that's the attacking mid (whichever kid that might be). The jersey numbers alter based on the kids, but the position numbers within US soccer are set so if the ref and the coach are talking about the 1, everyone knows they are talking about the kid playing goalkeeper.