It's not so simple. Even if you have learnt the alphabet song, the idea of sorting words alphabetically is not obvious, it's something we learn in school.

There are, in fact, two non-obvious ideas here.

The second idea is lexicographic sorting, which means that if two children share the first letter of their name, you compare them using their second letter, then their third letter. Sharing first two letters of names will very likely occur in a classroom of students. Even if it's obvious now, it took hundreds of years for people to figure this trick for sorting long lists of names.

The first idea is sorting a list in first place. This is obvious for our generations, because we have searched words in paper dictionaries and encyclopedias and phone books and indexes a lot, so we know it's easier to find a word in a sorted list. But I think this idea might not be obvious to a child today, when computers can find words in a book for you, can search names in a electronic phonebook or dictionary, or even just words in a list of entries you write. A second year child might never have used a paper dictionary himself, and have never had to find a book on a large row of library shelves with books sorted alphabetically by their author. (Even I have used the help of librarians and electronic catalogs so much that it wasn't until a few years ago when I first used paper catalog slips, and that I have seen a library shelf sorted by catalog numbers as opposed to alphabetized or Dewey descriptor.)

Please don't take this as a sign of generational gap. I think even in our generation a second year child might not have internalized how sorting works. It's one of the useful things we learn in school.